As I understand it, GeForce Now actually does require changes to the game to run in the standard and until recently only option of "Ready To Play". This is the supposed reason that new updates to games sometimes take time to get released on the service, since either the developers themselves or Nvidia needs to modify it to work correctly on the service. I have no idea if this is true, but it makes sense to me.

They recently added "Install to Play" where you can install games from Steam that aren't modified for the service. They charge for storage for this though.

Sadly, there's still tons of games unavaiable because publishers need to opt in and many don't.