Xbox has a digital marketplace on which game developers (sometimes a dreaded publisher) sell games and Microsoft gets a cut of the deal. Microsoft makes money because a game is sold on the Xbox digital marketplace. Microsoft has no (popular) equivalent to Steam, GoG, Epic Games Store, etc., and as such have no (popular) way of monetizing PC games sales.

(As in, we're still talking about a games market and not an OS market. OP doesn't have to rethink their claim that MS is "non-existent on pc" because they practically are.)