In the US, phds and professional degrees are more or less geared toward students who are comfortable enough financially to stomach the opportunity cost of 6-10+ years of additional education, unpaid or underpaid residencies and internships, and long apprenticeship hours (which prevent backfilling financial gaps) before making “real” income.
Can I ask why this is getting downvoted?
Most of the other comments are basically saying this ("the pay is too low for too long for not enough reward").
Anecdotally: I'm teaching a course in "How To Be Successful In College" (not it's real name) at the US community college where I teach Computer Science. I've got more than 1 student who are going to get a credential for nursing because there's just no way they can spend 8-10 years in school to become a doctor.
Would they be good doctors? The question is moot because it's never gonna happen.