Why bundled means worthless?

Probably they have a large supply of keys for that game.

People could easily stock up on keys during a sale and then just wait for prices to rise again. Apples get spoilt, but game keys you can hold onto as long as you wish.

People buying at discounted either definitely want to have them for their own, giveaway or want to use the bargain for a trade.
People buying a bundle often don't want every game in it. Therefore the game is more available in trade offers and on the grey market. With this high supply the price drops.
Also bundle prices are often even lower than discounts for single games.

Why if the game was bundled its value drops to almost nothing?

the game costs as much as people are willing to pay for it. If no one wants to buy bundle game for $10 ppl will sell it for $9 or less xD

It all depends on supply and demand. If the game got bundled a lot time ago but not many people are interested in it the price will be low, if there's still a bunch of keys going around the price will be low, if the trader wants to low ball the fact that it got bundled once will be used as an argument... you get the idea I hope.

Some traders want profit, there are some that make it obvious and it depends if the trader wants the game for themselves or not. If it was only bundled once 5 years ago, then they probably figure it's worth more or as much as it is on sale since it's been a long time. Add to the fact if it is popular or not and how much the trader feels it's worth and many price values differ a lot. Of course there are people who do not know or research if a game was bundled before or not and base it all on store value or personal worth. I think all you can do is post fair trades on multiple traders and hope for the best. If you find one that overprices too much for you, find another one.

In your example, it's entirely possible the 30€ is worth more(to them) than a non-bundled(game they don't want) 10€ depending how much the bundle cost at that time.

When buying, I've ended up mostly looking at stores that have exact pricing in $ or keys because that way I know exactly how much to match the trader for their games.

It's because one spoils, the other doesn't. It's rare for keys to expire after a set amount of time. If they do, then their price doesn't really change either.

Also, supermarkets tend to sell less of recently discounted items anyways. It's because they already bought those things during those discounts, which lowers the demand for a small period of time. Not to mention, one's a repeated purchase. You can't live off of that one pack of frozen french fries and those tomatoes forever.

I still have keys for 3 year old bundled games...
They're not worthless but they are worth less.

Comparing food to games, eh?

Lets say you want to buy an apple and there are shitload of bananas at this local store of yours all year long, next available store is 50km away and doesn't have a website where you could check if they have apples. So your local store is selling apples for 10$/each and bananas 1$/10 bananas.
Would you take a risk driving 50km to the random store, wasting gas money just to be disappointed or would you pay 10$ to get that apple you so bad want.
Your local store is doing bundles and that random store is gray market, which means you could potentially lose money trying to get something cheaper. Or you can just buy unbundled product, which is being sold at your local store - for extra price.

So the reason them being more expensive and more valuable is simply, money!

Prices are volatile and a product is worth as much as one is willing to pay for it, regardless of its nature or other synthetic evaluations.

You are comparing apples to videogames, and it doesn't work that way.

Apples will rot in 2 weeks, a key for a digital game is valid as long as there is a store accepting it (a Steam key from 2004 is still valid 13 years later). So, if you find apples with a 95% discount, you cannot haul them and sell them at a higher price 5 years later. With software, you actually can. You don't even need to provide for storage, refrigeration etc. It's digital. You can keep thousands of "digital apples" in a 1MB text file on that old floppy disk you're using to balance your coffee table. Also, unlike apples, you can actually ship keys for free to the other side of the world (even on the ISS, if they have a Steam-capable notebook).

Because digital copies have an unlimited supply that traders and resellers like to take advantage of while driving the price further down.
It doesn't matter how old a bundle is, if there are thousands of traders desperate to trade it for the first best offer it just loses all its value.