Why do people care so much about the 'color' of a person, I thought we all..I dont know..human?

When choosing a spouse, I mean I've talked to a lot of guys who would never date a black or asian or some random orientation girl(and some would be black or asian or some other ).When people say things like "your pretty, for a black girl''to me it just perplexes me. I mean I thought bone structure body and face were really important lookswise not the skin tone...I think its weird that in I dont know, Ashton Kutcher somehow got opposite of whatever skin condition Michael Jackson had, people wouldnt find him attractive..I thought it'd be the same.;/what do you think?