To get an insurer to want to maximize your health care, you have to guarantee that the insurer saves money if they make you healthy, and loses money if they don

To get an insurer to want to maximize your health care, you have to guarantee that the insurer saves money if they make you healthy, and loses money if they don't. If patients can switch insurers at any time, then an insurer that spends a lot of money on preventative care only sees a small fraction of the benefit; they lose it whenever the patient switches away. And if you spend 'too much' money on preventative care, you'll probably end up losing customers to cut-rate insurance providers with lower premiums. If instead we could measure health perfectly, then you could set up a system where if a person is made unhealthier by an insurer, then the old insurer has to compensate the new insurer whenever someone switches. But we don't really know how to measure health, if by 'health' we mean having a low probability that a person will become sick in the near future; we usually can only notice once a person becomes sick.