Because there is no genetic basis for race.
http://harvardmagazine.com/2008/05/race-in-a-genetic-world-html
Therefore preconceived notions about races being born with superior or inferior traits is wrong. Even when talking on average.
It's only the social constructs of race that cause these stereotypes to become self fufilling prophecies... Someone told they have the ability to do well in math usually identifies with math then and studies that way. Somebody who's told he's "people" are superior athletes puts greater focus towards being a superior athelte, and those who develop atheletes look for kids of that race, some consiously some unconsiously.
If you were to put children on and Island and let them be raised by researchers who were trained to present social stereotypes different from what they were... suddenly you'd see a bunch of athletic white people, black people who excelled at math more then the other races on the island etc.












