because they're afraid of thinigs they don't understand, and are afraid to confront the way they feel. Subconciously, they know they're wrong but they simply can't face that. So it makes them angry. And since they'll never take that anger out on the real problem (their ignorance), they take it out on what they perceive is the problem (other races).
Every person is different. For me, I would NEVER judge a person based on their ethnicity, i judge them based on their personality. Now i will admit i do make racist jokes once in a while, but they are jokes. My friends do the same things, they dont even care because they know i am joking. I hang around people i like and enjoy. My best friend Maria is mexican, my other best friend Dennis is african, most of my other friends are mexican, AND my boyfriend Rod is african-american. I have very few white friends even though im white. lol
I believe different cultures at first appear alien to us which intimidates us into making rash assumptions! It is a learned behavior! People look for faults in others to make them feel better about themselves or to help explain away something they dont understand! Call it insecurity maybe!