[belorn]

Carl Sagan wrote in his book The Demon-Haunted World about a kind of intellectual laziness.

"instead of judging people on their individual merits and deficits, we concentrate on one or two bits of information about them, and then place them in a small number of previously constructed pigeonholes. This saves the trouble of thinking, at the price in many cases of committing a profound injustice. It also shields the stereotyper from contact with the enormous variety of people, the multiplicity of ways of being human"

He talked about the issue of stereotyping in science and the harm this caused his field, but it can really be seen as an insight in why people so easy start to do it once the culture makes it acceptable. It is easier. It does save the trouble of thinking. Looking at people as individuals is hard, takes energy and is prone to come back and bite you. Much easier to just reduce people to single bits of information and follow what ever the cultural accepted stereotyping (ie discrimination) that the environment allow.