Doctors are highly specialized mechanics for the human body. The reason they can work the hours they do is because most of what they do is not creative and can be done on basically autopilot.

MDs also work in a profession with a fairly unique quality: you always get paid more for doing more work, and the work is more or less completely prepackaged into self-contained units for you - i.e. a GP seeing patients more or less breaks their day into a pattern of independently compensated, 15-30 minute slots.