Why is that so many people are judged ?

In today's society, just by looking at someone, people quickly jump to conclusions and always assume how a perosn may act and judge them by their appearance. If they actually took the time to get to know one another before judging, they would realize others are not what they seem to be. Do you know what it's called ? It would be considered a stereotype, Just because of ones nationality someone would have negative comments or assume what the person is all about, like if someone saw an Irish person, they would quickly assume, that person is a drunk and loves to drink alcohol, which may not be true. But because of the way society was before, like when segragation had occured and blacks were separated from the whites because of their skin color,and many whites judged african americans and thought they were superior to them. A stereotype is also so widely accepted by many so they continue to judge and it will always remain a problem in society but not as problematic as before, like when segregation occured or when hitler believed that his race was better then the jews.