But that's not the only/main reason. The proponents of affirmative action are guilty of the very thing they say they're against: racism.
When individuals are victims there's a system to deal with that. But you can't have "justice" for people based on birth, skin color, &c. You would have to have the same baseless criteria for discriminating against people. But instead of separating into "inferior" and "superior" you want to separate into "victims" and "culprits".
The solution is to treat the shortcomings, not the people. If black people are doing less well in school, then it might be that the real reason is that poor people are doing less well in school, and the solution would be to deal with that, not based on race.
You say 400 years, but the United States didn't exist 400 years ago. Are the descendants of Greeks (in the US) enslaved by Greeks owed compensation? Are the descendants of Europeans enslaved by Africans owed compensation?