I would almost call it a consumer enforced monopoly. Steam is simply so much better than the competition (besides maybe GOG) that a lot of people will not buy a product if its not on steam. How many people do you know that won't buy most games if they aren't on steam?
I sometimes buy games from non steam platforms but I put a lot more thought into it than I would when buying on steam. I simply trust steam to not fuck me when I know the competition will.
Same, I know installing from other launchers on linux is possible using lutris, but since the steam experience is so smooth, I've never bothered to figure out how.
If a game isn't sold on Steam, It isn't going to sell well.
Because of the huge amount of Valve fanboys who only buy games if they are on Steam.
Games on other stores might as well not exist.
And more, if a game even dares to launch on a different store, it would garner unfavour of all those Valve fanboys and hurt their sales even if afterwards it does launch on Steam.