Most people aren't really natural entrepreneurs or businessmen, and do best when they're employed by someone else (which is why most people work for other people rather than owning their own businesses). I don't see why doctors would be any different in this respect, as there's no reason to think that medical ability and entrepreneurial ability are correlated.

The same is true of programmers, of course. Although it can be hard to remember when reading startup-soaked HN stories, most programmers are happily employed by other people and aren't off founding startups.