It is not the professions that decide, it ends up being some politicians that decide to license this or that in a locality — and that IS a valid exercise of LOCAL governmental authority, albeit nearly always an abusive and harmful one.
Most often it is the professions putting pressure on the politicians to protect them from new entrants into the business.
for example, an insurance company, whether Health or P&C, that is dominant in a state lobbies with the state regulatory commission to structure the regulations in such a way as to prevent new entrants into the state that would increase competition. How do they do this? Creating complex reserve requirements the prevent out-of-state reserves counted the same way as in-state reserves, or creating licensing of captive agents different from independent agents, There are many examples.
The same is true of barbers/beauticians. When I get a haircut from an unlicensed person, the quality is no different than from a licensed person. The licensed ones, and especially the schools that get government grants and benefit from government loans to students, don’t like it and do their best to put the unlicensed ones out of business.