Doctors established them in order to preserve the legitimacy of their profession. That's my understanding, in any case.

In some cases it was to enforce a cartel (emphasis mine):

To hold him who has taught me this art as equal to my parents and to live my life in partnership with him, and if he is in need of money to give him a share of mine, and to regard his offspring as equal to my brothers in male lineage and to teach them this art–if they desire to learn it–without fee and covenant; to give a share of precepts and oral instruction and all the other learning to my sons and to the sons of him who has instructed me and to pupils who have signed the covenant and have taken the oath according to medical law, but to no one else.
...

I will not use the knife, not even on sufferers from stone, but will withdraw in favor of such men as are engaged in this work.

Oh, sorry, I wasn't clear. I didn't mean that such a rule existed, just that if one did exist, it would be ethical (in the sense of being a rule of professional conduct) and unethical (in a different sense of the word 'ethical') at the same time. Contrast the second definition on this page with the others.

Doctors established them in order to preserve the legitimacy of their profession. That's my understanding, in any case.

Well, many professions have established such rules, and presumably, they did so to make their professions more legitimate, as well as to give their members a guide to behavior their committees considered better.