When did we decide that someone who spends all their day doing hard physical labour will get paid wayyy less than

Supervision is a professional skill. A good supervisor is worth their pay. An incorrectly paid supervisor is never a lasting employee. Too much pay, the boss will realize the incompetence and replace him. Too little and the supervisor will seek a better check somewhere else.

Supervisory roles vary, and some are high demands and stress. Planning and execution of the daily workflow and designation of labor with the ability for contingency restructuring through the days shift overlaps.