Orphan Keys? Why?

The most gifters, sadly, don't care.

I think I mostly post keys in the mentioned thread when I got the key for free, or it doesn't give CV (or both), or I think the game has too low quality to qualify for a giveaway (in my opinion) and I don't really care if it gets played or not. It's not really a difference to public giveaways I think - many games you create giveaways for in public won't get played either.
And you never know if the ninja plays the game or not. Just because someone grabs a key in mentioned thread doesn't automatically mean the game doesn't get played. (EDIT: ok to be honest, that's probably a rare case)
A bit sad is that many do not say that they claimed a key. (maybe many are unregistered?)
The bots are indeed annoying (if it's true these keygrabber exist on here!) and keys should be (at least a little bit) encrypted.

Finding fault with someone for the way they give away something just doesnt sit well with me.

Some of the keys are offered there because people aren't sure they'll work. That was part of the point of the thread, I thought.

It makes me sad sometimes, but people can choose what to do with their own keys.

You're not wrong, but it's very unlikely the key will land in good hands. It's usually a place for collectors either way.

The main purpose of said threads is to drop keys that one either isn't particularly sure about, usually old stuff we're uncertain if it still works, or one doesn't care enough to put any effort into making a proper giveaway. And also for keys for other places besides steam that have a limited appeal in this forum, but those are more of an extra.
And I'm pretty sure that most of the comments saying "duped" or similar are meant to let know the user that dropped the key that it has already been used, not exactly complaining if you ask me.

But I'll agree that some sort of obscuring on the keys is preferable to just copy-pasting them as plain text and as such it should be encouraged, because keys as plain text only attracts bots and those are a nuisance.

If you have keys for games these you would like to see them played, you shouldn't giveaway them on sg at all. There is like 95% chance it will got to some autobot joiner, idle alt farm or collector who won't bother ever start it at all.

There are some users who ask who wants the key and the first one who answers gets it. But not everyone does this, and I respect their decision because it is their key, they decide what to do with it. Then others try to encrypt it in some way, but anyway, there are people who take it and do not thank or maybe they were already used and did not know. I believe that those who do not worry about encrypting it do so because, in the end, there are users who take the keys and are not able to thank.

I'd risk that most of the games on SG go to people who won't play it to begin with.
You don't need to be a bot or ninja for it

Shakespeare called it "casting pearls before swine" just saying.

Being aware that I'm repeating what others have already said, and without taking your question as critical of the process that I (and others) choose to use:
I very much use that particular thread for keys that I am not sure are valid (either due to age, or if I am not sure whether they've been redeemed and so on). For those I would much rather throw them out in a situation where people/bots/aliens/etc. know there is a chance they are dud, rather than get someone's hopes up too much. If I've got greater confidence in the keys I'll use another approach.