Why do humans judge others by looks?

As long as humans exist, this will always be a problem. You can ask anyone that is literate about the saying "don't judge a book by its cover," and they will most likely know that means. At the same time there will always be people who judge by looks cause they feel like they are better than the other person. Another word for this explanation is racism. Why does it even exist? Because people like to put themselves over others.