While I tend to agree, insurance companies don't see it that way. They need a doctor to indicate a necessity to treat a condition, as opposed to it being the easiest way to treat it.
For example, I have to take digestive enzymes to digest my food (pancreatic insufficiency). For someone with an unusually high metabolism, they would also give them a leg up on gaining weight, even though there are other approaches to gaining that weight. However in many cases, the insurance company wouldn't cover their prescription when they will mine.
As always it’s insurance nonsense. If incentives were aligned insurance companies would be lining up out the door to give this to obese people because they (the insurance companies) would eventually be on the hook for paying for the care of the conditions you just mentioned. It is very well demonstrated in literature that obese people have a much higher occurrence of these conditions than non obese people.
But the system is not set up with aligned incentives