Apparently Teeth, Eyes and Minds Are Not Relevant to Health

I went to the dentist this morning.  The was nothing wrong, it was my semiannual cleaning.  Like most people who are lucky enough to have dental insurance, my dental insurance is not the same as my health insurance.  I have a separate insurance card for the dentist.  Mine happens to be the same provider as my health insurance, but I’ve worked at places in the past where they used different providers.  It costs extra to get dental insurance.  The same is often true of vision care and mental health treatment is often simply not covered.  This makes no sense.
In all three cases, severity can move an issue from its traditional realm to a medical condition.  If a person needs a filling, she better hope she has dental insurance.  But, if a person is in a car accident and fractures several teeth, it will be a dentist at the hospital who treats him, but it will be covered by health insurance rather than dental.  If a person needs corrective lenses, hopefully he has vision care.  But if a person has glaucoma, her health insurance will cover it.  If a person wants to go to a therapist to get help dealing with the death of a loved one, she is probably paying out of pocket.  But, if a person has schizophrenia, he is covered by health insurance.
MDs and insurance companies should know better than anyone that everything is connected.  Gingivitis increases the risk of heart attack.  Poor vision contributes to accidents.  Depression contributes to poor diet and a sedentary lifestyle which leads to obesity.  MDs ought to be doing everything they can to encourage their patients to see dentists, optometrists and psychologists in the interest of keeping their patients healthy.  And insurance companies out to encourage the same in the interest of profit.  It is way cheaper to pay for a cleaning every six months than it is to pay for open heart surgery.
I understand that dentistry, ophthalmology and psychology have their own histories and traditions outside of traditional medicine.  I’ve heard that the reason they are treated differently is essentially that the American Medical Association is really snobby.  They believe only MDs are real doctors and they want to keep the fake doctors separate.  I have no idea if that’s true or not.  It seems ridiculous, but if it is true it wouldn’t come close to being the most ridiculous thing ever. As for insurance companies, I can’t even hazard a guess why they treat dentistry, ophthalmology and psychology differently from other areas of medicine.  I would love to know, so if anyone has any insight, please share.