Problem is that not everyone can just pick up and move, even doctors. Independent practices take years to build and overhead costs in civilized blue states are higher, even if you form a partnership with other physicians. Most doctors don't start a practice until their early 30s, so they're fortysomething by the time they've paid off their massive student loans. And that's assuming they're financially responsible, which many MDs are not. The siren song of buying a Benzo, or a Porsche, or some other luxury toy to reward oneself and keep up appearances is strong.
Fact is, even successful people who make six figures put themselves in situations that make moving cross-country difficult.