Why doesn't an osteopathic physician have an MD degree since he is a doctor who practices medicine?

The MD and DO degrees are earned after graduate education at a medical school. Since MDs go to a allopathic medical school they earn a MD degree, while osteopathic physicians go to a osteopathic medical school and earn a DO, which is a Doctorate in Osteopathic Medicine.

The MD and DO degrees are equivalent doctoral degrees, both of which are recognizable for licensure as a physician/surgeon.

One can compare this concept to the fact that there are also equivalent degrees in other fields:

Dentistry: DDS and DMD are the two doctoral degrees recognized for licensure as a dentist.

Veterinary Medicine: DVM and VMD are the two degrees recognized.