These guys have great arms cause you are looking at the top guys in the world at their sport. You have to be a genetic freak to reach these levels. Sure the gymnastics build their arms, but if these these same guys trained for bbing and never took gymnastics they would still have great arms, cause they have the genes for it. Most gymnasts never reach their level, and never get arms like that, cause most simply dont have the genetics for it.
Its all genetic