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.