What happened with game bundles?

I think it's a combination of many factors all playing against each other. At first they were a great way of exposing some indie games to the world. And these were top quality games. Then demand for more like it lead to more sites popping up with bundles, but there weren't enough quality games to fill all of those, so the quality of the games dropped along with the price gamers were willing to pay per bundle. At the same time our libraries were exploding, so it meant it was getting harder to find new games to entice buyers with, especially when developers were starting to realise that once a game was bundled there would be key resellers stocking up to sell their game at a hugely reduced price, and their own profits would suffer, especially if the bundle site had no checks in place. So more developers stopped putting their games on the smaller bundle sites.
While this explosion of bundles was happening, steam greenlight meant there was an explosion of new games on steam, many of which were poor quality and cheap, so it was easy for smaller bundle sites to fill bundles with these. But then steam started restricting the number of keys small developers could ask for and also stopped games from counting on achievements and game count, making these cheap games a lot less desirable for game collectors and the like.
Any one paying attention during the peak of the bundles era could see this coming, as it was just not a sustainable model. What started as a great idea, and which benefited all parties, quickly became a feeding frenzy of greedy gamers abusing the bundle sites for profit, greedy developers dumping their junky games and shady bundle sites stealing keys or not paying developers. Frankly I'm still amazed that we even get bundles of any sort of quality these days.

I have no idea how it was on golden age, I came back on PC games world just 1 year ago, and my feeling is that Fanatical and Humble still offer interesting bundles, even if it has been better by the past (if I understand) . To nuance, Fanatical platinum bundle doesnt hold my attention since months.

Otherwise, why "obligatory" gift? Is it a condition to open a debate? Anyway thanks

Here: (https://www.reddit.com/r/humblebundles/comments/8xd4fz/how_humblebundle_created_the_bundle_market_and/)
The short answer, many bought a bunch of bundles only to sell their bundled key separately for profit which hurt the publishers' profit. Older bundled keys are still dirt cheap on key shop sites.

Keyshops happened, and other grey market sites. Even steamtrades, that I use nonetheless, hurts bundle sites. People complain about IGN as some explanation, but the truth is Fanatical does rebundles all the time, and literally none of the other bundle sites matter. And it all happened after traders/grey markets sprung

Resellers. It wasn't enough that people got a bunch of games dirt cheap. Some had to turn a profit and ruined things for everyone else.

Aside from that, bundles also killed themselves. Once you see a game has been bundled, how likely are you to pay more for that game? This makes devs wary of bundling their games.

Developers realized their games worth money. They are featured in bundles less, and if you check prices, many sales aren't as deep as they were a few years ago.

Groupees always was a small site and they likely made a few bad decisions.. more and more greenlight bundles, bundles with unknown games that started getting worse and worse, they were behind on delivering bundles after trying a "season pass" for Be Mine bundles.

Indiegala had and has some nice deals in the bundles, but for most of the time it always was for collectors and hoarders - getting many different games very cheap.

Fanatical and Humble had some drop in quality as well - Fanatical mostly due to repetition, Humble a lot in weekly bundles, and some in monthlies (They aren't surreal good deals, but usually good quality games at a good price). I was always curious how they get developers to get their games (that cost let's say $15, and are on sale for $6-7) in bundles that usually average out around $1-2/game.

Also... it's hard to make new and original games. Even through lovingly created by a small team, a 235th retro-inspired pixel-platformer with RPG elements isn't just the hit as similar games were 5-8 years ago.

edit: and of course as others said, resellers. Bundles themselves can be bought only for a while, but the keys persist on the market, usually way below the store's price.

Greenlight and later Steam direct made it too easy for "developers" to release their junk on Steam. While we experience an avalanche of releases there aren't enough games which bundle sites consider marketable and customers are willing to pay for. Thus we have an increase in re-bundled games than new-to-bundle ones.

A lot of reasons that come on all involved levels down to:
"People are greedy"

I think a lot of bundle sites failed/disappeared/got worse because it's usually always the same games being bundled, so once someone has them there's no incentive to buy any more bundles because they already own the majority of the games. You know the types of games I'm talking about.

The signs were there from the early days that things were not going to last. But to understand why, a quick history lesson might be in order.

Humble bundle launched just shy of 11,5 years ago, and it was a smash hit. A small number of great indies crammed into a bundle sold at a price that was very hard to beat. These days we would probably consider Humble Indie Bundle 1 to be pretty mediocre, but when it launched it was something fresh and interesting, and discounts on Steam were not as high, or as frequent, as we would get used to either.

The developers who put their games in the early Humble bundles also reported great success. Not only did they earn a bit from the bundle, but their games also got a big indirect marketing push, as people were reporting on the bundle and about the nice games, and several developers stated that they saw a sharp increase in sales on Steam for months following the bundle. While the earnings from the bundle itself were nice, that was not were the true value of being in a bundle was.

With the success of humble other parties were taking note, and you got a bunch of bundles that were all trying their own spin on the formula. Buy games not Socks did a single bundle where you got 5 pretty good games for $5, Indie Royale did its weird community pricing where people paying over the base price would lower the price for everyone and Indie Gala were straight up copying Humble. While buy games not socks would only get a single bundle under its name, the other two would turn out to have some staying power (though Indie Royale would go down with the Desura ship, as it was owned by them). With new bundle sites popping up the indirect marketing push was becoming less and less of a thing though. With Humble Indie Bundle it was easy to stand out, because there were not that many other vying for attention, but now things were getting crowded. But things had not gone terrible yet, and bundles were selling well. In fact, at this point, for many smaller developers, bundles were really their best bet at earning money. This also coincided with an increase in discounts on steam, both in terms of frequency and size, as well as how soon they would happen. Now people in the industry were starting to take note of a worrying trend. Several developers were starting to talk about a "race to the bottom", and how the market was becoming unsustainable. They were basically training people to wait for steep discounts or bundles, and these were coming very soon after launch.

Steam was also changing. Greenlight opened up the possibility for a lot of developers who previous could not get their games on steam to start selling their games on the biggest PC digital distribution platform. And the bundle sites become a marketing tool for games wanting to get through greenlight. Groupees, Indie Gala and Indie Royale all went in on the Greenlight train. While Greenlight allowed for a bunch of really good games to reach a new market, the complete lack of quality control also meant that there was a lot of dreck. And people were voting for anything they thought they could get a steam key for, which meant that the Greenlight bundles were quickly filled with mostly junk games that would have no real chance of getting through greenlight on their own merit. Trading cards were the next thing to influence the bundle market in a major way. While in its early days Trading Cards were only available on a select few games that were given the thumbs up from Valve, this would change pretty soon, and Valve would allow any game to get trading cards. Bundles were starting to pop up that were only really catering to trading card farmers. A fare few bundle sites would go bust during this period, bundle sites that did not lean into the lucrative greenlight and trading card craze, that actually tried to have some semblance of quality (it was pretty tough for sites like Bundle in a Box to get much attention in such a crowded market). With so many actors on the market being in a bundle really meant nothing when it came to discoverability either, so the thing that that the indie developers who were in the early Humble Indie Bundles were reporting on was more or less gone at this point.

Parallel with this a big grey market was growing, where keys from bundles were starting to be sold. It was not something that happened overnight, of course, and really, key trading happened from the get go with bundles, but the grey market was becoming more organized and reliant on bundles. And now developers were starting to report that they were seeing a sharp drop in sales from the point where they added their games to a bundle. It was the exact opposite of what happened in the early days, where being bundled would see an increase in sales. Being bundled basically meant that for a long time after the bundle ended it would be hard to sell the games through normal means, because so many people were turning to the grey market to get their keys. There were other issues with the grey market beyond the sales of bundle keys, but that's beyond the scope of this topic.

Around the time when Humble decided to launch their monthly bundle things had taken a sharp downwards turn when it comes to bundles. This was 6,5 years ago. Monthly was probably an easier sell for developers as they were not feeding the grey market to the same extent (harder to buy multiple bundles), and the mystery nature of it also meant that people would be more likely to spend money on a bundle in the hope of getting something good, and then find that they would mostly get games that they would not really have been willing to pay a high bundle tier price for (despite Humble Monthly being sold at a price similar to a Tier 3 in a humble).

With Valve realizing that trading cards were becoming an issue, as it encouraged developers to just quickly dump garbage games onto the store and then sell the games for pennies outside of steam they started to clamp down on trading card games, and this hit the bundle market hard. Several bundle sites would go bust over this, as they would no longer get an influx of no effort games that they could sell for cheap (remember those bundle sites that gave you the option to buy 100 copies of a bundle at a discount? Those went away). A few other bundle sites were scraping along, but with no trading cards, no greenlight, and a generally bad reputation as junk mongers, it was becoming harder for them to find any developers willing to put games in their bundles. The people running groupees made a comment about this when they had that Christmas bundle were they could not deliver the promised number of games until a month after it "should" have ended, and they just kept extending and extending the bundle, scrambling to find anyone willing to work with them.

Hopefully you can see that there were really several important factors that would add up to the bundle market becoming deflated. After a meteoric rise in the early 2010's, the market ended up becoming unsustainable, and for the last 7 or maybe 8 years there's been visible problems with the market. The market did not crash as fast as it rose, but it's been a slow decline.
Going along the deflating bundle market has also been a chance in attitude from developers and publishers regarding discounts. We're not really seeing games get 75% discounts within a year anymore, not unless something went wrong. It's generally taking far longer for games to be sold for cheap, and the massive discounts are usually reserved for older games. Developer started talking about the effect an early discount could have about de-valuing their products, and it has even ended up with some companies ending up in short-lived feuds over it (remember when Epic had their first $10 coupon and Paradox removed some of their games from the Epic store? Paradox claimed that Epic had now set a new expectation for the price of some of their games, one that was lower than Paradox wanted it to be). In a climate like this, bundles are not as good of a fit as they used to be, and games are increasingly being treaded like long-term investments, rather than something that gets thrown out onto the market and then becomes irrelevant after a year (took them long enough to realize that digital distribution changed how long a game could stay relevant....Games no longer have to worry about the shelf space they take up in stores, so they don't need to just rely on the massive sales peak of the first two months, they can continue to earn a profit for years)

greed, not only from bundlers but also from customers

Too many bundles, not enough games. When bundles became a big thing, they had a huge pool of games to work with. Didn't really matter if they were a few years old or not, but now that most bundle customers have built up their 1000+ game steam library, that available game pool has become much, much smaller. Hence the repeats.
I think publishers actually make decent money from bundles. There's so many customers that wouldn't have bought their game at the price the bundle sites are paying, but no publisher is ever going to want to consistently put their new releases into bundles. That's simply bad business.
In the case of HumbleBundle, IGN also shaves more off the top than the previous site owners did. Manipulating the BTA and higher base prices to start with.

Like many others said, it's complicated and there's many factors, but one is that Valve, because of many exploits, limited the amount of keys devs can pull nowadays. Before, anyone could just ask for 100.000 keys and then 100.000 more tomorrow and so on... Many of those half a decade old keys now float on re-seller websites... but today it's much harder to get keys even to participate in Humble.

Here's an example I saw earlier today: https://www.reddit.com/r/gamedev/comments/sqpiik/valve_refuses_keys_for_bundle_i_signed_a_contract/

There are over 50,000 games on Steam last I checked, so there's no shortage of potential games to bundle. The problem is convincing developers to sell their game for almost nothing, and then killing future sales potential due to excess keys floating around in the grey market. Most developers are now wise to the fact that by bundling their game they're likely killing its long term sales.

I think the grey market mainly killed the bundle scene and Valve is ok with that. There was a feature on Steam that allowed bundle sites to add directly a game to a user account instead of generating a key but Valve got rid of that. They want any game in a bundle to end up the grey market hell where no game comes out. And that's the reason the same games got bundled forever, once you're in hell you can't go deeper and this is also why so many developers don't want to be in bundles anymore.