I do think it's really goddamned funny that nearly every single other big game publisher that's tried to compete with Valve has failed miserably, due almost entirely to their own incompetence(s). Origin, Uplay, Battle.net, et cetera, all total flops that nobody was ever happy to use. Not because Steam is an excellent service, christ no; even at its best, it's a store with frequent sales tied to an annoying DRM launcher, and not because they (to my knowledge or memory) even really lifted a finger to try and do anything about them, but because apparently Valve is the only major games publisher with any foresight past their next quarterly profit report and knows how to actually run a business sustainably.
network effect plays a big part too. steam isn't a library manager, it's a drm manager, so it's not like you could bring them somewhere else.
"another game launcher? Why couldn't it have a steam release" is because steam has locked away your embarassingly large amount of money spent on games, on their platform, and made management of those games kinda miserable even then. But no one can match it, because it's one of the largest vaults of human sunk cost