why are stereotypes always mention in our society?

Stereotypes are always in our society beacuse thats the only way we know how to deccribe a person.For example lets say we see a african american we immediately think they have no knowledge about anything or live in the worse neighborhoods when really they can be the one of the smartest people and live in a safe environment.Another example can be that just beacuse some white girls have blonde hair are so stupid and can be called "bimbos"but then again they can be really smart.Also theirs alot of people that stereotype puerto ricans beacuse they are so loud and called them "mira-miras" when really thats not nessecary.So basically stereotypes can be like judging a book by its cover.