Because generally building your body and knowing hard work improves the mentality of those who do it. They are often more stable (minus roiders), disciplined, and even conservative. These are the little people. Shunned by society, probably have terrible family lives, picked on in school, ect. They have terrible self esteem, and their minds are chaotic. This comes out in their outward appearance as they display their inner chaos on their shoulder, a visual display of rebellion against the world they hate so much. Their self destructive and nihilistic tendencies show.