He is playing “real guitar hero” not fake guitar hero or simulated guitar hero. Playing the guitar and playing guitar hero are not the same despite containing similar looking elements. In one case the target is to produce music, in the other it’s to produce points or a high score. He doesn’t care about producing music, he cares about producing a high score. Likewise, the musician’s aim isn’t to be better at scoring points, it’s to be better at producing music.
To use another game, one could say “why play Madden when you could play ‘real’ football?” Madden IS real. Is it football, no. It’s a game. But that’s OK. Football is a game, Madden is a game, but they are different games. The aim of playing Madden isn’t to become the next great linebacker for the Chicago Bears, it’s to be better at playing Madden.
Chess is an example outside of video games. It’s said that chew is a simulation of war. But people don’t play chess to become better generals and generals don’t play chess to become better at winning wars. They are each their own thing.