The basic answer is that they are two distinct health professions that aren’t all that closely aligned and they operate under separate systems from both a health and economical model. In others words, it is that way because it has been that way for a long time.
It is a good question though. Dentists used to have a reputation for being the ones that couldn’t hack medical school so they became dentists instead. That isn’t true anymore for the ones that I see. My dentist’s office is right across the hall from my primary care doctor’s office and she is a cutting edge medical professional who runs a tight ship. There isn’t any difference in the way the two offices run except that my main doctor takes care of 95% of my body and I get sent across the hall to a whole different system if the problem involves the mouth. The insurance is vastly different however even when she does surgery which doesn’t make a lot of sense as you say.
There are ENT’s and oral surgeons that work in the mouth area that are doctors just like there are opthamologists that will do some of the same things that an optometrist can but the insurance plans are different. The medical professions aren’t always laid out in clean-cut divisions between roles and the insurance schemes add more complication to that. It doesn’t make much sense. I can see an acupuncturist or a chiropractor under my medical insurance but they won’t pay if I see dentist for anything even if the problem could result in general infection, jaw damage or heart problems later.
We have threads on the reasoning (or non-reasoning) of this before. I will see if I can find one.