A very good point. Put it this way, no matter how many number of guitar hero games they make, their still going to sell loads. Businesses want to make money and that is what Activision is doing. Guitar Hero is a perfect game for this. It's the same thing with Fifa.
I've never bought a guitar hero or rockband game myself, mainly because they release a new one so often and it cost so much for the guitar and the game when in comparison to a normal game.