Because most of the NBA are black, well-proportioned and slender, and they are all very tall, so you think their muscles are beautiful without comparison.

You think their muscles look better than those in the gym mainly because their muscle fat content is low, so they look unhappy together. They are the kind with a sense of lines. The same is true for white people, like Phelps. I think his muscles are super beautiful because your body fat content is very low.