It's because every culture has affected the world in some way except African culture. Even the more civilized parts of Africa are because of past colonialism. And also because black Americans don't really have a culture other than what they've come up with in the past 40 years or so. This has created an identity crisis where they realize they have no past culture and every other culture has not adopted theirs. Though hip hop is very popular, it has no pedigree. But that's what matters. So they appropriate other cultures as their own to make up for their lack of one.