Cuz they make $2 a month and drive a rotted 57 chevy?
Partially because their salaries are less. Mostly because they are practicing medicine rather than playing CYA with legions of Tort Lawyers, Insurance Companies and Federal Regulators.
They don't employ one or more secretary/clerks to handle the paperwork for the above ankle biters.
The medical care I have received from indigenous doctors in Africa and Central America has been quite competent and more apropos than what might have been rendered by a US doctor unfamiliar with local diseases. These indigenous doctors all had a comfortable, though not excessive, standard of living in relation to their local economy. Similar to small town doctors of the 1950's & early 60's.