The doctors I know, despite the toil and suffering, really, really like being doctors. They like what they do for other people. To a one, they are more than capable enough to jump into any other career, likely more profitable (when seen as a ratio of time/money or pain/money at least).

This isn't true of all doctors, but you'll probably find that ones that suffer through like what the author (Dr. Levi) discusses find that their practice is a calling, not an occupation or job. As a calling, it's part of their identity, giving up on it just doesn't make sense to them.