AFAIK Apple prevents this by forcing every browser app to use the half broken Safari engine that does not implement many modern web APIs and standards.
Making a first-class gaming experience will require considerably more than this on iOS, and as far as I am aware, Safari's implementation is spotty at best.
My company spent years getting a passable Safari/iOS mobile web 360 video player working, a whole game like Fortnite would be another beast entirely.