Millions of people bought Guitar Hero and Rock Band to enjoy a fantasy of being a rock star. I don't think anyone bought these games expecting to actually learn how to play music.
Have you learnt to become a space marine from playing Quake? Have you learnt to rule a nation by playing Civilization? Have you learnt to draw graffiti by playing Jet Set Radio? Have you learnt to be a hand-to-hand combat master playing Street Fighter?
Unless a game is explicitly designed as a teaching device, you are not likely to learn anything more than a vague caricature of the skills involved in the activity it simulates. Nobody expects to learn any activity based on a video game. But somehow, people constantly criticize Guitar Hero and Rock Band for this.
Why do you think anyone expects these games to teach them to play a real instrument?