Why do Rap and Rock get such bad stereotypes?

I listen to a lot of rock and metal. I went on the rap boards asking why people liked rap. Through all of the flaming and insults, I finnaly got a very good answer. He explained that the rap that I had heard was mainstream, and just like mainstream rock, it sucked. But one of the people talking bad about rock said"rock is just a bunch of devil worshippers in black who scream real loud about death." I kinda laughed, because that was so far from the truth, but then I thought I probably look the same way to him. So, I'm asking, how did these music styles become so misunderstood? Sorry if the question was so long.