Because comedy provides a veneer of safety around an issue. The hahaha part of human nature is largely about reacting to being uncomfortable. People avoid thing

Because comedy provides a veneer of safety around an issue. The hahaha part of human nature is largely about reacting to being uncomfortable. People avoid things that just make them seriously uncomfortable, and comedy provides a safer, lighter way to deal with it. It also makes it harder for opponents to attack. It's really complex. It's nothing to do with vulgarity, vulgarity is just common in comedy because it's another thing that makes people uncomfortable.