Most doctors don't have as strong a profit motive as a dentist.

Most people see a PCP that doesn't really have the means to recommend you for thousands of dollars of treatments that he'll oversee. Instead he refers you to a specialist, but you still have your PCP to rely on to say "Hey, this specialist says I need surgery for this, is that reasonable or should I get a second opinion".

But when you go to the dentist and he says you have 15 cavities and need two root canals immediately and he can get you an appointment as soon as tomorrow to get started, a lot of people will just do it without seeking a second opinion since there's a lot of friction to finding a new dentist and undergoing a second exam.

When I needed a root canal, I self-referred myself to an endodontist both as a second opinion and for his expertise at doing the procedure. My dentist's dental tech seemed surprised I wanted to go somewhere else for the root canal, she said "Well, most people like Dr Xxxxx do it, he's very good at root canals".