We're not all one human race. Humans are not a race. Humans are a species, divded into races. That's how racism can exist.

Ethnocentrist is way harder to shout illogically at people when they aren't actually being ethnocentrist. Racist is short and sweet, easy to say.

Racism starts or exist when one race feels superior to the others. Many factor...and mostly physical appearance (eg. white or black), culture, especially with concern to language, behavior, customs, and religion. And summing up in general the experience being with certain people from certain race. Ethnocentrism in general.

Racism exists for the same reason people argue that two apples are not whole units. The most minute differences between people will undoubtedly cause racism. Even if the difference came down to hair color.

We aren't the same, it's just an illusion. There are huge differences in our capabilities, ability to learn, intelligence, aggressiveness, goals, emotions, value systems, life styles. It's not just about "the race", more like society you come from...

The old style racism is based on fear from unknown, often comes from ignorance, you may see that the most racist places/societies are the least developed ones.

Imagine that you put together a random person from slum near Mogadishu, Saudi prince and Professor from MIT.

They are all humans but they are very different people.