Why are some people against race mixing?

mixed people only make up(all mixtures) 2% of the american population. you can look it up, yet we have contributed more than blacks, although some biracial have NO self-respect and claim to be black, which is sad. like Obama, alicia keys, mariah carey, ect. those girls are NOT black, why would anyone want to be considered black? I just don't get it. It is absurd, they should be seen just as biracial. how can some people be against biracials when we on average contribute more than anyone else including whites. however since the white population is bigger(75%), they have contributed way more, but biracials being 2% regardless of the mixture have contributed a LOT