Human laziness "causes" stereotypes. Stereotyping is, by definition, applying the characteristics of an individual from a group to the entirety of a group, regardless of it's accuracy or actuality. Sometimes a person does fit a stereotype, but that's more coincidence than anything. Just because a stereotype is occasionally right doesn't mean it's ok to propagate it. Many stereotypes aren't even based on truth.
People like stereotypes cos it saves them the trouble of actually having to learn individuals as individuals. It's far easier to just say "these people are like this". Also, a lot of stereotypes are actually just racist propaganda or rooted in bigotry from times long past. The world is still a lot more xenophobic than it wants to admit, and it's easier to point to a stereotype and justify one's thinking than it is to actually overcome one's internalized biases.