Because in other countries with higher degrees of socialization in education and medicine, many of the costs that American students/physicians are responsible for (tuition, malpractice premiums) are partially absorbed by the system.
Assuming everything else stays the same as posited by OP, it would literally be impossible to practice medicine in certain parts of the country without additional funding once you take into account the cost of medical liability, loan repayment, licensing/licensing exams (which can average out to at least a couple hundred dollars a year if you divide up the fees), overhead, etc. This isn't even including the opportunity cost of spending the best years of your life making -250k+ while you're in undergrad/med school and earning a pittance while you're working "80 hour" *wink wink* weeks in residency. God help anyone who wants to be fellowship-trained anything in neurosurgery.
If you're going to push physician salaries towards the scales seen in "socialized medicine," you better hope the rest of the system gets pulled along with it. Even after the crapshoot that was preclinicals and Step 1, I love medicine more than ever.... but I'm not going into a profession where my salary is arbitrarily capped, all while I'm still at risk for being sued for money I don't have and trying to pay back loans with money I don't make.
https://www.health.ny.gov/health_ca...s/2011-10-17_medical_malpractice_premiums.pdf
http://www.amednews.com/article/20100503/profession/305039938/4/
Obviously those are some extreme examples, but I can't think of too many fields that are expected to not insignificant portion of their salary on malpractice/liability insurance (even if we assume a more benign figure like 5k for low risk specialties, that would still be 5% of your yearly income @ 100k).
If you can find a way to squeeze in cup ramen into your budget on top of a mortgage and the kiddo's college bill, you probably should have gone into accounting.