Because Stadia offers an inferior experience to local rendering. In particular, the way the video compression crushes all dark colors into a single shade of black. Plus, when Google kills Stadia in a year, you'll have lost the games you purchased and paid a subscription fee for.
I was excited to see Google make a game that would only be possible on Stadia, be it an MMO, races with hundreds of cars, or something more creative their designers could imagine. With their internal developers shut down, all that's left is more expensive, inferior quality versions of games I could play elsewhere.
Honestly, I'm sick of the limitations of traditional gaming. If that means some compression quality loss and being somewhat inferior to local rendering, then that's the price I'm willing to pay. Playing games is not one of the most important parts of my life, but with the amount of crap I've put up with dealing with flexibility and compatibility issues of a superior local rendering setup for modern games, you might think it was.
Why? Probably less latency/better quality with a PC.
Obviously it depends on how well your local hardware compares to Stadias server farm too.
The PC and Stadia versions are much better than the PS4 and Xbox One versions though - those do not run well at all.
I suspect the actual PS5 and Xbox Series X versions will be relatively similar to the PC/Stadia version, but they haven't been released yet - you can only play the last gen console versions via backwards compatibility.
edit: in what way does Steam or Epic expect you to understand how to use a “gaming PC”?
And as the other commenter points out, playing steam games via geforce now doesn't even require installing the steam app anyway, it's just an account link.
By considering Steam and Epic, you are already looking at this from the perspective of a platform. I am simply saying Stadia is better if the game you love is already there. NFL Madden is the latest Stadia promo right now. If most of your gaming time is consumed playing Madden, Stadia is probably the way to pay for it.