Laziness.

The barriers to entry are high: 4 years of undergraduate education, 4 years in med school, 3 to 7 years of residency. Those years are very expensive, with the average med school graduate student piling up $250k of student loans. And if you’ve ever watched a friend go through med school and residency you’ll know it’s life-consuming; they are immersed in work and miss out on years of socializing and culture. The payoff at the end had better be commensurate or nobody will be a doctor.