This is my understanding of the US situation.

It used to be that doctors were running their own small medical offices a lot. Unfortunately, due to insurance overhead it's no longer profitable for individual doctors to do that, especially without existing patients, so the remaining offices are mostly from doctors near retirement age.

So as doctors retire and new doctors don't want to start their own practices those practices have been getting bought out by large medical networks or private equity. Some offices are also going out of network with insurance companies. It doesn't help that PE is willing to pay a ton more than any new doctor could for the practice.

I'd recommend picking medical plans with out referral requirements and with good out-of-network coverage.