Because we also have the highest levels of consumption, of pretty much everything, including a lot of health-damaging things. And it's insurance companies that are required to cover preventive care, not doctors. There could be better incentive alignment. For-profit insurance company profits are capped, so the more they spend on care per unit time, the more they can charge and profit (though realistically, profit is very much the smaller part compared to even the rest of the non-medical expenses, averaging 2-4%, out of the 15% non-medical expenses allowed), but that doesn't explain the 60% of large insurers who are non-profits nor why Medicare and especially Medicaid will do so little in the way of encouraging preventive care. Medicaid has exactly the incentives a national program would have, and yet it does next to nothing to encourage its use early for preventive care.