Humans in general like to generalize people and put them into groups. This allows for issues to be looked at more simply, although mostly incorrect. This leads to stereotyping, racism, sexism and other actions where people are grouped based solely on opinion. And no, it's not just white people who do these things, practically all humans do.

It is really unfortunate because this starts a cycle. As people look onto others expecting them to be something they are not, they can change into it. For example, people looking onto Muslims as terrorists and bombers, some people can just be like "**** it, that's what you want? I'll give it to you". It is a terrible cycle.

Plus, people also like to feel superior in a way, putting others down based on these ridiculous stereotypes is a way for them to feel above someone, or in this case, a whole race.