Because doctors have a financial incentive to order and perform/give expensive procedures and drugs that may not necessarily be medically necessary.
This is obviously a somewhat different situation, but I'd remind you that lots of doctors made a lot of money by unnecessarily prescribing Oxycontin that the spiraled into the opioid crisis.
It's not unreasonable for there to be some kind of check, though to be clear, I'm not saying the current system is good. But, insurance just automatically paying for anything a doctor orders is open for abuse, and that needs to be addressed one way or another.