I get what you mean, and yeah they already have it, it's called an iPhone, but there is a reason why I own a Nintendo Switch, an iPhone simply just isn't enough, and gaming on a touch screen isn't ideal, I can buy accessories.. but meh
The physical experience is not the same, and the software experience is also not the same, a dedicated device would give proper signal to game developpers to make/target/re-release their premium games, rather than designing games to be played for few minutes on a touch screen
That kind of accessories usually map physical buttons to touch inputs on screen, not idea
Similar to movies, that's not because you can watch movies on your phone that you'll want to watch all your movies on your phone, a dedicated device is often what you want