Probably b/c you aren't normally able to. It gives the people a chance to play as something you usually arnt able to. So let them. Don't questions. What does it matter to you? Huh?! What what. In the butt. Ya.
Edit. I think the last game that gave me a chance was mass effect 3 yet I still didn't take it.
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Almost every game gives you the option to be a chick. So most guys will create em and create their ass n tits as big as possible.