There are some in academe who believe that all professors should be paid the same, irrespective of their field of specialization. A little bit like yours truly wanting to be paid as much as as a brain surgeon because we are both “Doctors”. By title? Yes. By skill set? No. Even though, in some strange way, we both mess with people’s minds!
The tenets of differential compensation lie in the fundamental principles of economics — demand and supply. If you want to make more money, seek education and training in a high skill, high demand, highly competitive field, and the dollars will flow. Otherwise, it’s just a case of sour grapes.