Undermining the power of software vendors is an institutional imperative at Apple. There is a memory of the days when they were dependent on Adobe and Microsoft for their hardware to be viable. When they design App stores they make the rules and game the system with this in mind.

It's not just that the stores are open to everyone-- shovelware and all. Steam does that but because they care about the ecosystem they protect pricing for premium products. They make reviews and recommendations relevant. Try to get your terrible knock off of a hit game come up in a search-- they are on to that.

It's notable that other attempts to develop game app stores for non-console platforms have fallen flat. If Microsoft has gotten any traction at all for game distribution on Windows it's because of the really different GAME PASS model. Blizzard, EA and such have apps to download their own games but don't challenge Steam for third parties, Good Old Games with it's anti-DRM stance is the only real competitor.

Steam is a model of integrity and it's a good thing that it's not for sale because it would be an obvious acquisition for irrelevant players like Gamestop who want be relevant today, it would have been a better acquisition for Microsoft than Activision but any acquirer would kill it one way or the other by violating its integrity.