Most indie games I've play don't even run their own online infrastructure because of costs. Why bother, when you can just use a storefront's matchmaking for free? And storefronts provide it as a means of soft lock-in. For example one of my favorites, Deep Rock Galatic, doesn't have crossplay between the Steam PC version and the Xbox PC store version of the game.

And there's already software to emulate Steam's matchmaking because it's so common.