Playing a guitar is hard. It takes impeccable timing, hand eye coordination, discipline, and tons of practice. One incorrect hand position and a masterpiece can be off key or sound like a middle school band concert. And even getting to that point requires a lot of technical know-how (learning to read music, how to create chords, etc).
Playing guitar hero is easy. All the complex positions are reduced down to 4 buttons and a 'whammy' bar. Learning all the music notes (treble, bass, quarter notes, staccato) is replaced by a tele-prompter.