Medicine isn't only meant for the money. Some people like the status, some people have had personal experiences and said "wow, I know what I want to do," and for the unfortunate, some just decided that they need something to do (nothing wrong with that). The money is a nice incentive, but for anyone to say that is the only motivation to be a doctor is a complete douche, because you can find other less stressful jobs that pay the same (and less work to i.e. the education process as well as residency and fellowships and whatnot). Why go into something extremely difficult if you can go into business or politics and make the same? For those going in for the money, they are sacrificing their youth to do something that will give them a large (but not huge) payoff.
My dentist, a very good one at that, who has her own practice, drives a beat up 1998 Honda Civic. But on that token, that is what we see. How about when she gets home?!?
But the car a doctor drives has little to do with what kind of doctor they are. If I recall correctly, people of all professions drive Mercedes and other expensive cars.