There are different status levels among doctors and among nurses, but a PhD in nursing stands on the other side of a clear border from a beginning MD. Similarly with lawyers and paralegals. These dichotomies stem from licensing restrictions, which in turn are descended from medieval guild practices.

Are you sure you understand what these professions do? There's probably less overlap in the skillset of doctors and nurses than you think. A doctor can't do a nurse's job without some training. Might apply to lawyers and paralegals too, but I don't know.

You didn't consider that the selection process for training is stricter for high status jobs, and so one should expect these people to be better decision makers. MD is vastly more difficult to achieve than a PhD in nursing.

There used to be this shorter training program in Finland for nurses who wanted to become doctors. The program was quit because almost nobody graduated.