Because they don't know it yet. They audit and review each claim a an office sends, which starts a negotiation process to see if each claimed item by the physician was warranted and done properly. All that information is codified then into a level of visit, which changes the amount it will be payed out.
It is grotesquely complex, but the core issue here is that patients want to consume their healthcare through insurance, because it is tax advantaged. Thus it is impossible to make a cash business that would cut through all this bullshit for the majority of healthcare.
Congress has been considering transparent, prior authorization legislation in the realm of CMS. I don't know much about it, but they did publish an RFI recently[0]. I hear it has bipartisan support (that's obviously hearsay, but a lot of this recent healthcare legislation that has been passed has not been divided along party lines, AFAICT)
It is grotesquely complex, but the core issue here is that patients want to consume their healthcare through insurance, because it is tax advantaged. Thus it is impossible to make a cash business that would cut through all this bullshit for the majority of healthcare.