Why is dental school a separate thing from medical school? I have always found it peculiar that terminal degrees in medicine (in the US, at least) are awarded in two forms. On the one hand, we have the MD (and the DO), a holder of which can go on to practice all sorts of medicine, ranging from pathology to radiology to oncology to family medicine and so on. On the other hand, we have the DMD and the DDS, holders of which can go on to practice oral medicine, and, as I understand it, little else. 

Why is this so? What is it about oral medicine that has it being taught in different institutions than all the other branches of medicine? 

(I don't know how this stuff works outside the US; answers contrasting the US system with other countries' systems of medical education could be very informative.)
the dentists professional society regulates entry into the dentistry profession for the protection of the public and the maintenance of member income.

the doctors professional society does the same.

neither one is interested in admitting the others students to their respective profession.