People like to have quick ways in order to process information. Because of this, stereotypes have been constructed based on certain patterns which people use to generalize anyone who fits the requirements of a stereotype. I agree that this is a bad thing to do because people are complex beings that can't be understood through generalization.

I think this happens because people usually fear from the mystery. So when we see person we don't know or barely have known we tend to put him in the nearest character we know just based on his appearance, then this improves to judging people.