Because guns are alot of fun, they help us defend ourselves and shoot for recreation. It's our right as free men to be able to defend ourselves from crime, nature, and corrupt government. Heck if I wasn't cheap as hell I'd own one.
Besides kid you could kill someone with a pillow (Smothering ring a bell?) or a cast iron skillet. Or a hammer. Do you want those to be illegal to.
Edit: Don't wanna risk rudeness