We already have. It turns out that patients generally don't want this. When given the choice, they generally opt for the practitioner with more training (the physician) over the one with less (an NP or PA).
Sure, we can (and do) pass these costs on to the patient, or (in some cases) force them to use the cheaper option, but at that point, you're literally talking about either:
(A) forcing patients to assume the costs of their care directly, either entirely or in proportion; or
(B) forcing patients to use the lower-cost, lower-quality option
Politically, people don't like (A) because it means patients' access to quality medicine is restricted by their ability to pay. And people don't like (B) because nobody likes to be told that they can't have access to the top level of quality (whether or not they're expected to pay for it themselves).