correct me if im wrong but a semi automatic is a gun that you don't have to reload between shots while an automatic is a gun you don't have to let go of the trigger
thus a revolver, many rifles, or many shotguns and stuff like that are neither semi nor full auto
anyways the answer to the question of why semis are allowed is just that they're convenient. whether for sport, hobby, or crime that's just it. i shoot a range as often as the bullet cost allows me and i gotta say bullpups and stuff are pretty damn fun to screw with
of course that's also the reason you don't see people with muskets in their basements but you know
