Why are people so closed minded?

Okay i've noticed a trend going on, especially on ask/answer engines like yahoo answers. When someone asks a question dealing with race of any sort, someone has to bring up a stereotype. And someone happens to reinforce it. I don't want to get too specific but generally, I don't understand how someone can fully believe something (a stereotype) without actually having proof. Somehow people can take the word of others (or scarily just make it up) and convince themselves that its 100% true.