Side note: Valve isn’t doing the thing Unity tried to do. Unity tried to charge you every time someone installs the game. And you’re not even hosting the game’s data on Unity’s servers.
Steam takes money when you purchase, then will let you download it for free, anytime, anywhere, and on any device. Completely different.
Back on topic: It would be really interesting to see the actual server and bandwidth costs for hosting and distributing all those games. There’s no way it’s super low, or any of the competition surely would have caught up by now.