It happened for so long, at all levels of society, that it is still affecting us today. It has shaped the cities we currently live in. It has shaped our language. It has shaped our laws.
The first girl Black girl to go to a school that was all white is only 71 years old. And that was barely the start of the end of segregation.
A bad example, because I'm only American, but anyone in the European Union could move anywhere in the European Union. Some places just have depressing weather, or are very isolated. But it would still take a great effort for the people who even want to move to actually move.
If you ever see little kids with different skin tones playing together, they are clearly not bothered by skin color, and it has no bearing on their play. The racism in America is completely learned. It is not natural. Sometimes consciously, but many times unconsciously.
Loosely similar is how men in most places are fine walking around alone at night. While women consciously try to move in groups, even during the daytime. Many men have no idea how different it is for many women.