It's an insightful contrast to draw. There are a number of differences between dentistry and other health professions, but I think the most relevant differences are the size of the typical firm together with the total cost for any particular patient. Many dental clinics are just one or two dentists, who have no business relationship with any other clinic. (There are "fast-food" chains like Aspen Dental, but they are a relatively low portion of the overall industry.) Also, although some patients might get low-five-figures worth of dental work in a particular year, that much is fairly uncommon. That same sum could be
one day in a hospital, which is usually owned by a firm with thousands of employees. Lots of formerly self-employed physicians have been forced to join big firms to retain admitting privileges etc.
Dental insurance firms would probably like to engage in all the price-fixing and patient confusion that other health insurance firms enjoy, but they can't possibly employ enough people to arrange their dastardly schemes with tens of thousands of different dentists. Instead, they have to publish (to dentists, not to the public) lists of reimbursement limits by procedure code. Dentists decide whether they'll accept any particular insurance plan largely based on reimbursement, and many of them are willing to discuss those decisions with their patients. At the same time, most dental patients don't have insurance at all [EDIT: and many are fine with that, since they can afford the care they need].
Single physicians in family practice would be in largely the same position as the typical general dentist, but the work they do is a relatively smaller portion of the overall industry. After all the real screwing from medical costs takes place via hospital visits and high-tech imaging. Even a smaller hospital will start getting calls from insurance executives after they generate a few expensive cardiac procedures in any particular quarter.
Don't take this as a defense of dental insurance. It's fine if you get it from an employer, but after working in a dental clinic I don't think anyone should get it privately. If you need a lot of dental work they don't pay for much of it (this is also true of many employer-provided plans). If you don't need a lot of work you end up paying them more than you'd pay the dentist. Most people who need lots of dental work but can't afford it should get loans for that work, from e.g. Care Credit or one of its competitors. Yes, that sucks, but it sucks less than private dental insurance.