HaKatan: the difference is that doctors will (usually) ultimately pay much more in taxes than a Rabbi will. They take because at that point they need it (and that’s only if they actually take – most places give them a stipend to live on. At least that’s what I’ve heard from 3 friends of mine who went to medical school).
Most guys in kollel are not using it towards their parnasah. Out of the thousands in BMG, how many become Rabbonim, Rabbeim in yeshivos, etc? Probably not even half.