I never understand why classify people on their looks. Talking primarily about the US. There is such a wide spectrum of white people coming from different cultures. Also, you're considered Asian if you're a Chinese immigrant but also if you were born in the US to Chinese descent let's say. The main common thing between the two groups is looks. Same for Africans and African Americans who are both classified as blacks…
Because it's easy. Our brains are expert pattern matching machines. The things we see immediately cause our brain to act and match based on the prior data it holds. What's the easiest thing to see about a person that relays the most information to our brain? The color of their skin, what they're wearing, their hairstyle, etc.