I imagine there is too much churn for insurance companies to be able to be confident that they will be the ones to benefit from lower long-term health costs and not Medicare or some other insurance provider.
This is right. I worked in health insurance for a while. If you aren't in Medicare Advantage (which has relatively lower plan churn) it is very unclear that preventative spend on an employer based plan will benefit the payor that paid for it.
Not for any specific payor, but when people shop around insurances and every insurance pays Ozempic, then everyone will profit from it in the end.
But they'd also benefit as recipients of these "churning" patients. To abuse a saying, "A slimmer population lowers all insurance costs."