Why do people have to generalize others by their race?

A person is an individual. So why do some generalize black people as having tons of kids and being illiterate and on welfare. Why are white judged as having and living in trailers. Why are hispanics generalized as being "dirty" and why are asians supposed to be "nerds" Of course there are large majorities who may be these things but I don't understand why there's so much hate and bigotry. I know people of all these races and some are stand out wonderful people and some are not. People are individuals and should be addressed for who they are, not for the pigmentation of their skin.