Why is rock music, seemingly more than any other genre..?

So obsessed with darkness and death and stuff like that? I mean, I know there is a lot of it in Blues, Hip-Hop, Country perhaps (only in a whinier way), but Rock music has so many bands and genres that revolve around things like that (Black Metal, Death Metal, etc.). The look of lots rock bands now usually is very sort of morbid or grotesque, and in no other genre have I seen such obsession with things like Satanism and evil. Why is this?