it takes a long time for societies to put those major rifts and social wounds behind them. several generations.
imo, racism is essentially a cultural problem. a group of people stays in one place long enough, their genetic evolution makes them a distinct race. they develop a distinct culture which becomes part of them and their racial identity. then you have them interact with another group, and even if both groups are generally good people, if there are strong differences and lack of common ground, they will likely become alienated. then if they're still forced into proximity with eachother that becomes conflict, and over generations the specific kinds of conflicts between cultures may become associated with that race, so they form racial attitudes. if these attitudes become solidified into a self-protective sense of superiority vs. inferiority, then, I think that is where it becomes racism.
on the plus side, every new generation has kids with a blank slate that can rewrite their attitudes and look for common ground and build a collective culture and racial identity. but these kids have to fight against previous generations that have even built political platforms based on the old racial attitudes; even minorities can have the attitude of 'we are not them, they are not us' and that can inhibit a cultural renovation. so it's this subtle ubiquitous attitude generated in cycles by cultural dissonance, that is the real issue.
and it's very problematic when people oversimplify it