Because they're racist, narrow minded and fear anyone who is different to them. I'm fine with black people, I had a black lover for a while until my mum screwed it up (NOT because of his race, she isn't racist, but because he was an university student and we didn't have a lot in common, plus us having sex within an hour of meeting also put her off, and it would have with a white guy too). There's only been one black person I didn't like (I did initially because he was hot, but he turned out to be a dick who swore at me and was addicted to crack).
It's all about the individuals involved. I'm more interested in why you post about "people hating black people" on a regular basis. Both questions have the same answer - no one knows.