While most doctors go to medical school and get an MD [1] before specializing on some aspect of medicine, in some subfields people commonly specialize earlier and have an alternate school. Dentists are the best known of these: you go to dental school for a DMD or DDS, learning in detail about the mouth and surrounding areas. While dentists do learn some about the rest of the body, it's much more limited than the pre-specialization training an MD goes through. Other subfields that specialize earlier are optometrists (OD), and podiatrists (DPM). [2]

This mostly makes sense to me: if someone's going to be focusing on one specific section of the body, shouldn't their four-year post-graduate education take this into account?

(My impression is that this division has worked well. If there's indication that dental/podiatric/optometric patients are getting worse care because their doctors have specialized training I'd be curious to see it.)

One place where the current situation doesn't make much sense to me is with psychiatrists. They go to standard medical school, get an MD, and then do a residency in psychiatry. This means they have to learn lots of specific medical techniques completely irrelevant to psychiatry, and even go through surgery rotations. Perhaps creating a "Psychiatry Doctor (PD)" degree and associated training program would be better?

[1] Or a DO. I had thought that osteopaths were trained very differently from medical doctors, but Wikipedia indicates that most of their training is the same, including that they both attend medical school.

[2] I think PharmD (pharmacists), DPT (physical therapists), and DNP (nurse practitioners) probably don't go in this category. Reading a little it looks to me like those are cases of the professional organizations pushing to name practitioners something with "doctor" in the title so they would be higher status and get paid more. (They do involve additional education over the older lower degrees given in those fields, but it still looks to me like quite a bit less than you'd get as an MD, DMD, OD, or DPM.)