I don't think the "should" plays any role here. In my opinion, paramedics, nurses and doctors should all earn shit ton of money; so should miners, and janitors, and garbage collectors, and other people doing dangerous and/or socially necessary jobs. And entertainers should earn way less than they do. But here we are.

Supervisors earn more because, through the way the job market works, they can ask for more. That's all there is to it.