We literally have the science, and, over a population and over time, no, they per se do not.

The things that do mostly aren't accessible to normal people (three or four professional nutritionists and trainers and doctors helping out several hours per week; moving to a skinnier country) so "diet and exercise" has been the only available course to recommend, and it's better than nothing, but it's depressingly ineffective.

This is precisely the difference between could and does I was trying to highlight to address this kind of thing in advance: diet and exercise could work, like, on paper they work every single time! Hell, technically, they're even essentially all of the mechanism by which methods that do work, achieve their effect, just indirectly. But just trying to improve diet and increase exercise directly does not work, not over a population. For outliers, yeah, but for most, no. Improving those is a an effect of approaches that do work.

The answer has been mentioned in other parts of this thread:

They simply don't feel the hunger that obese people do. Some people eat a larger than normal lunch and then won't feel hungry at dinner time and will either just have a snack or skip the meal entirely.

Others, like me, can put down a 22 oz steak and then still feel hungry again just two hours later.

It's not a matter of habits, it's about what their body tells them. My body is always telling me to eat even after eating an irresponsibly large meal, and ignoring those signals difficult. Those people that are in shape don't have that problem. Their body is telling them to eat less.

Why they can't, I am unsure. Is it because hunger is a stronger drive than long-term planning? Is it because food is, like many other products and services sold in a capitalist economy, incentivised to develop in maximally addictive ways even without a deliberate choice by any supplier? Is it because advertising really does influence people? Overuse of antibiotics makes people hungry? Something else entirely?

Whatever the cause, people can't apply those methods. Which means they don't work for most people — not that they can't work for most people, they just don't.