Great Z's blog has hit the nail on the head.
People basically are not altuistic. Everybody has some motivation for what they do.
A number of years ago a national society on whose executive I sat, had Patch Adams talk at their annual meeting. This was at the instigation of a Big Pharma company who offered to pay for Patch Adams, take him or leave him. Because I was on the executive I happen to know that Patch's fee was greater than $10K. Not in Bill Clinton's or Wayne Gretzky's ($99,000)range but still not bad for one hour's work. Like many people I saw the movie with Robin Williams which by the way, the real Patch didn't like. One of the issues Patch mentioned repeatedly during his talk was that he only earned $400 per month. While it is possible that most of his $10K+ fee went to some worthy cause, I'm sure Patch has somewhere nice to live, eats okay and is generally well taken care of.
Why did I go into medicine? My father was a professional, it was instilled into us early on that we had to be professionals. Other professions didn't appeal to me; it was medicine by default. One of the biggest draws was not the money but rather the fact that even in bad times, I was likely to be employed. I make a nice living, I am more or less happy. A lot of people I work with or know make less and are equally or more happy.