There are plenty of cases for exercise curing disease. Metabolic syndrome is basically cured exclusively through diet and exercise until it becomes too late to stop. Physical therapy is exercise and certainly that is curative. Exercise alone took my cholesterol panel from mildly elevated to absolutely normal, lowered my blood sugar, and improved many inflammatory markers. I cannot say the same for naltrexone. Bonus, my joints feel better, my lungs work better, and my CRP has never been lower.
I dont even need to link studies. Literally go to the NIH, search title for "exercise" and whatever you want, and I bet you can find enough repeatable evidence to completely change your mind. I'd argue exercise, 4-6 times a week in some form or another, is arguably the simplest cure and preventative "medication" for the largest variety of common ailments in a post-scarcity society. The fact doctors don't actually "prescribe" exercise as a first line treatment for metabolic diseases is really a failing of our drug-happy medical system.