But the fundamental causal mechanism at play is very simple.

Sure, calories in vs calories out... Except it is not helpful when you cannot effectively control one or both without reducing the real or perceived quality of life to the level where people refuse to exercise this control. This is where most diets eventually fail. And you seem to agree with that, while still maintaining that "understanding what needs to be done to lose weight is simple", where it is anything but, since it includes understanding of the actual doable actions one has to perform and still enjoy life. And this all-important understanding is sorely lacking in a general case.