A few reasons. Firstly it's unfair to a good person or a skilled person to treat them as if they are not; or conversely for Progressive racism, to pretend the bad is good. I care about fairness and that means treating people based on who they are.
Secondly, for simple practicality. I am capable in a few moments of interaction of judging how easy someone is to get along with, if they're intelligent or not, if they're shifty or not. Most of us have these basic capabilities of human interaction. So going to some media-informed proxy process is both unnecessary and in fact less accurate than the evidence of my own eyes and ears.
If someone wishes to quibble with those positions it's likely that they want to talk about tendencies and generalities. Well that is the racism - judging the individual based on other people who share some racial group. You can talk about race (though discussions are often ill-informed messes) without being racist, but it's racism when you treat an individual differently based on that rather than who they are. It's neither fair nor, usually, efficient.
Thirdly and finally for now, humans over-exaggerate pattern recognition. This is a known thing. In particular negative reinforcement is known to outweigh positive reinforcement. Add in the tendency to conflate culture with race (especially Americans and Progressives love doing this), and it's usually just not well-grounded. People, of any skin colour, are just typically not well grounded in rational thinking, and that's something we all know is true.