Why would you think these things? The road to becoming a doctor in America is simply more competitive than any of those groups, and the selection at every step is theoretically predominantly merit-based.
I studied philosophy in undergrad, philosophy honor society membership, 4.0 philosophy GPA, the whole 9 yards. Most of my professors were extremely intelligent (and one of them even told me, near graduation, that I would have made a great colleague which was very flattering, but anyway . . .)
That said, there was a lot more variability and I'm certain that on average they are less intelligent than physicians. I had one philosophy professor who could speed read a passage of Hegel they hadn't read before and immediately explain precisely what he meant (trust me, this takes a tremendous amount of verbal intelligence and logical aptitude, as well as intuition because, IMO, Hegel is just kind of a bad writer in some ways). On the other hand, I had one professor who was simply obsessed with the technological singularity (i.e. OMG ROBOTSZZ!!?!) and spent his research time writing pop philosophy books about "Philosophy and [Television Series]."
Honestly, in most academic pursuits, it's completely possible to make a career out of passion and a proclivity for some random esoteric ****. A lot of these people are probably intelligent. I just don't think it's strictly required like it is in medicine where merely being an above average student with an interest in the field doesn't get you very far.