It was a deliberate decision that was made to make it highly impractical to actually do. GeForce now has over 2000 games available to stream - do they actually expect Nvidia to submit 2000 apps? From a dev-infra POV that’s obviously largely impractical. Would Apple even allow that?
It’s also a baffling from the user’s point of view. What happens a single game is available on multiple platforms? I search for “Destiny 2” on the App Store and there’s 3 different listings for it on Stadia, GeForce Now and Xbox GPU.
Would it be reasonable for Apple to require Netflix to submit each movie and TV show to the App Store, and then say "no no, we do allow streaming tv!"