Everybody has biases conscious and unconscious that have to do with our upbringing and life experiences. The human brain naturally forms patterns and this includes people. If grow up with a mom that always asked your dad to fix things, then you might have a tendency to believe that women can't fix things and men can. It's hard to see people as unique individuals.
We see people - gender, race, how they are dressed, etc. We can't see essence, so the superficial things form our judgement.