The main reason? Those of us that do not believe in racism whatsoever are still aware that it tends to creep in subconsciously when you're not paying attention. So we still have it too, we just try to fight it inside ourselves to be better people, instead of just being instinct-driven.
Like, I was treated very poorly in my youth for race reasons, so not being racist has always been important to me. Yet since I never grew up around any black people, only really saw them on tv or when we drove somewhere, I experienced some trepidation the first times I was around them a lot as an adult. They're just different. Talk a little different, act a little different, etc. I wasn't afraid per se, just uncomfortable. I had to consciously work to process that to get rid of it.
Anyways though, I'm quite certain there's still some subconscious biases floating around down there. I deal with them when I notice them.
Are you saying humans are inherently racist? I disagree; I think some people are just raised that way and retain those beliefs into adulthood due to confirmation bias
I believe humans are inherently racist, or actually discriminatory against those not considered part of our "tribe," whatever that may be. We have a built-in "us versus them" mentality that exists despite conscious efforts to educate and be mindful of those attitudes in ourselves. The sooner we admit to subconscious bias, the sooner we can mitigate it by remaining vigilant against that kind of thinking.