My assumption is that dentists, optometrists, podiatrists, do not deal with life and death situations. For instance, dental clinics are typically not open 24 hrs a day since most cases can wait until the morning. Doctors on the other hand need to be on call all the time in case of gunshot wounds, heart attacks, and etc...
This is especially true as people have less cavities and dentistry seems to be shifting to the cosmetic side.