Why do we still have affirmative action?

I understand that racism is still a huge problem, but how is affirmative action helping the ethnicity divide? Doesn’t it inherently differentiate between white and black people, drawing a line between them? Wouldn’t choosing students based purely on their academic achievements and not at all based on their origins or skin color be the best ‘non-racist’ way of going about it? This results in people getting ahead of those that are more qualified than then just because of the color of their skin. How is that contributing to a post-racial world? Basing any criteria on these factors is inherently racist.