Treatments like exercise also have high variance - depending on the circumstance and how diligently you implement it, it may have no effect, a big effect, or even cause injury in the worst case.

Insurance likes low variance outcomes because they’re easier to model accurately

I’m not in the insurance industry and have no idea what the actual reason is.

And I don’t know what the statistics are, but a significant percentage of people with gym memberships rarely go to the gym. I suspect the percentage of people with free (i.e. payed for by insurance) insurance who rarely go would be much higher.

>health insurance doesn't cover gym memberships or exercise equipment

Because neither of those things are necessary to exercise.

I bet if you asked your GP for some body weight exercises you could do at home he'd happily provide some for you and insurance would cover the visit.