I posted some time ago on the difference between playing and performing music, with Guitar Hero being an example of how you perform the role of being a guitar player rather than actually playing guitar. (Slash professes to not being that good at Guitar Hero).
In Rock Band things are a little different: Drumming is actual drumming (your physical actions map to being an actual drummer), singing is actual singing … but the guitar is still about performing the role of a guitar player.
Why is that?
It seems to boil down to the different parts to play – almost everybody can sing, if badly.
It is very simple to be a bad rock drummer (bass drum, snare drum, bass drum, snare…) that still fills the part of a drummer, if badly.
By comparison, guitars have pretty high barriers to entry – if the song has a fast riff, you can’t get away with only playing every other chord. And then even playing guitar chords takes a while to learn (and makes your fingers hurt).
It’s the instrument. (Perhaps that was obvious, I just needed to write it down.)