But nobody's using Stadia, that's the problem. Even after the service became free to use, nobody's buying anything on it. That's why it needs first party games, something you can only get on Stadia.
Stadia isn't a store, it's a platform because its running on its own hardware with its own services and features.
They're not using it because they think the service and/or prices are shit. Games aren't going to change that. Like I said, you might get people to play 1 amazing game on Stadia, but if they still think it's shit then they'll only play that 1 game and use other services for all of their games still.
Steam runs on its own hardware and has its own services and features. Still just a store.