And the fact that they work directly to save/improve/alter life whereas great software projects tend to be somewhat disconnected from the actual life saving thereby making their authors somewhat removed from that 'thing' that allows doctors to charge the big bucks.
You know there's a lot of medical software right? And I'm not just talking about record keeping. What do you think most high end medical and scientific instruments run on?
I'm just a programming but I think a large portion of the bucks surgeons make is due to the expense and duration of medical school.