Because I am simply awful at plastic guitar.
I wince/chuckle at the kids saying how easy this game is - "medium" is about as much as I can manage, and not all that well at that. I have been at this since I bought it on the 9th, and in that time I have scored 100% on medium guitar on exactly nothing. If I dare hard (let alone expert) I am usually done within 5-10 seconds. I can't even play "Yellow Submarine" on hard. The good part of this is that I can look forward to many hours of play without worrying about having "beaten" the game, because I don't think I ever will.
Someone needs to make a game with keyboards. That I would ace.