That is still the insurer saying they know more about the medical situation than the doctor. The doctor is the one who is equipped to decide what is necessary.
The doctor is not infallible. He or she is likely extremely busy, and under many pressures, e.g.:
- patients who adamantly insist on X treatment, make a fit and threaten a bad review, even though they don't need it
- fear of malpractice suits (e.g. 99.9% chance treatment is unnecessary and a waste of money, but in the 0.1% case I might get sued to oblivion if I didn't prescribe it)
- intense lobbying from pharma companies who spend boatloads of money trying to convince them to prescribe their products
In general, they don't directly pay the costs of using limited healthcare resources, but they can pay serious costs for failing to use them, so their incentives are skewed.
Our current system is far from ideal. But a system where a single person gets to make all the decisions, while foisting all the financial burden on someone else, would collapse within a week. Someone has to be the bad guy to sometimes say "we can't afford this, sorry".
... which many would argue is between a patient and their doctor. We dont pay premiums for no reason, and the insurance company isnt really allowed to determine what "necessary" means.