How does it apply to only rock bands?

And yes I agree rock bands sound the same, so do teen bands, same can be said for hip hop/rap, so do indie hipster music, any type of music really. Each genre of music is going to sound like the genre it was intended to be and hence groups sounding the same. Individual preference comes into play with liking certain bands but the general sound of them all is quite similar because it is supposed to be otherwise it would not be considered that type of music. Not sure what you were trying to get at.