You're not seeing the other side of this. The HMOs and insurance companies have ridiculously low rates they pay doctors and dentists. The docs then charge higher amounts to the insurance companies so they get a higher percentage of what they really think it should cost. If a doctor tells an HMO procedure X costs $500, the HMO says fine, here's $250. The doctor loses out. So the docs charge the HMO $750 and get back maybe $350.
Sometime ask your insurance company to see what the doctor billed and what the insurance company paid. You're entitled to that. You'll be surprised how little of what the docs charge that they actually get paid.