Dieting and working out definitely does work, the problem is that the median person attempting it will badly yo-yo over the years while feeling terrible about themselves and probably not really getting that much healthier over the long term. So it does work, but it also doesn't, practically at all, for the overwhelming majority of people who attempt it. That's why a lot of these posts end up having people talking (well, writing) past each other: diet and exercise does work. It works great. It's also a miserable failure that's nearly useless.

Again, even those with extensive and expensive outside support see depressingly poor outcomes on average, though of course that does improve things somewhat. Those are still a ton worse than GLP-1 agonists, as far as efficacy. And that's the very best effort we've got for "diet and exercise" interventions, short of live-in dietitians and chefs and personal trainers or putting people in total institutions.

Meanwhile, people move from a skinnier country to a fatter one and usually get fatter. Willpower wasn't what was keeping them skinnier. It makes no sense to expect willpower to be what'll make the fatter country skinnier when that doesn't seem to be why skinnier countries are skinnier.