None of what you just described is preventative care from a doctor. Everyone knows about exercise and junk food that doesn’t mean someone will chose to do or not to do those things.

This is simply not true, to a significant degree, as people age into and past middle age. (And even that is ignoring that accidents happen to healthy people of all ages. Eating well and exercising don't eliminate the need for expensive medical care after being in a serious car accident.)

I don't think the point is that eliminates every single cost, but offset some very major costs. Hope you see and obesity derived illness are real.