Because half of them care more about their looks then they do about their games. Notice how Serena is still dominating because she actually has a good muscle mass and therefore can generate a lot of power? If Sharapova and Vika actually bulked up and stopped getting pedicures, maybe they wouldn't get spanked off the WTA GOAT all the time.
I dont think they need to necessarily bulk up. I think they need to just train even harder. Its not that difficult thats what they have trainers for right? But i dunno now that the courts slowed down everything looks all fucked up. *sighs*