In life, you are RARELY paid for the skills you bring to the table. You are paid for how in demand your profession is vs. how crappy your profession is and how much it will take to convince someone to take that job. Once you understand this basic fact you will be far less frustrated with the apparent randomness of salary differences.
The physician profession is a bit different because they are often self-employed and there is a complicated web of reimbursement and overhead to consider. This sort of "crappy job pay" does extend into the extreme working hours of surgeons and OB-GYNs but otherwise it's almost impossible to predict what a typical physician will make because of the nature of reimbursement and self-employment.
Professions that are employed by other people, like many psychologists, PAs, NPs and RNs, will be paid whatever it takes to retain their skills. I often hear people complain about how much RNs make despite the fact that they might only have an associate's degree. The point is not how many years of education they have...it's how much you have to pay someone for them to want to keep what is often a very unappealing job.