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