Why isn't extreme body building considered a mental disorder but anorexia and others is?

Anorexia and muscle. Excellent question and observation! the reality is that both conditions share the following, body image disturbance, obsessive-compulsive behaviors, physiological disruption (anorexia leading to cardiac problem etc) (body building use of steroids etc). The problem is that anorexia is more life threatening requiring medical attention. If body building is an illness what would happen to its industry.
Ask the DSM committe. The psychiatric diagnoses are formulated by a committee and it is a endless series of compromises. Extreme body building to the point of compulsive exercise could be diagnosed.
Maybe it is. Some may consider extreme body building as a type of body dysmorphic disorder where an individual sees his or her body other than it is and continually tries to change it.