Are they Steam exclusive - as in they refuse to release on any other platform - or are they only on Steam because it's the largest and most convenient place to release a PC game?
In case of Valve games: Yes absolutely.
In case of many other games: Also yes.
Arma 3 became steam exclusive because it's convenient for BI to have just one platform they need to update, which is true. A lot of "steam exclusivity" is not because it just happens to be the largest place (it's def. not the most convenient place, that's itch.io). It's becuase Steam held the monopoly on PC distribution, and not being on steam was death. That is it's own problem, remember when
EA games were removed from Steam by Valve? And yes, they have been let back on the platform now. But it needed an Epic bombshell for that to happen.
But why should developers not choose for Epic? Having your game on Epic exclusively is more convenient as well, and it earns you vastly more money. You not only get paid for the privilege, your game sells more.
I don't like Epic because it splits my library, and I'm concerned about its longevity. If they do lose the war, what happens to the games?
I 'like'* Epic because it provides me with an alternative in case Steam drops out. Steam held a monopoly on games distribution, what does happen if they sink for whatever reason? If I put all my eggs in a DRM-enforced hellhole, I'm screwed if it ever falls over. Much like I was actually screwed when I did that, and Steam had a very unreliable offline mode. Remember that, with the exception of GOG and Itch.io, these platforms are all about making DRM appealable to the masses. Should they fail, you're ****ed, no matter whose platform you're on.
I don't like the way they've bought so many exclusives.
Yeah it's inconvenient.
I don't like the way they're mostly owned by a chinese corporation.
They aren't. Why would this be a problem?
* I like talking about this but also these are just DRM-enforcers that try to sweeten the deal of signing away what I consider to be basic consumer rights through free stuff or massive discounts.