This is because their job absolutely requires performing on insufficient sleep. Combat isn't shift work.

I think (without much data here) that medical training pushes insufficient sleep on doctors due to on-call schedules. While hospitals have doctors, many of their specialists come from private practice or multi-specialty clinics, which rotate being on-call between then (often in 12 hour shifts). The other cause of insufficient sleep is that if a patient under care of doctor has complications, they will often be called, even if just for a consult.

Economically, this is likely efficient compared to having hospital specialists (which may end up with nothing to do many nights), but causes docs to train for worst case scenarios sleep-wise.