Rooster, I do have a theory on that - because dental care was not always so expensive. I am not sure when the shift happened from reasonable to skyrocket-through-the-roof, maybe 1990 or so?
But I think it is (partly) because of the advent of dental insurance. Formerly you expected to pay for dentistry yourself, but then insurance came in so the dentists could raise their rates in keeping with MD's!
I just found this on the ADA website..."The average net income for a private practitioner in 2015 was $179,960 for a general practitioner and $320,460 for a specialist."