There's anecdotes, and there's science.
Individually, there's (previously) been nothing better to suggest than "try harder (and, maybe, smarter)".
Statistically it was almost useless, but it's the best we had. It's not bad advice exactly, it's just extremely unlikely to work for long-term, sustained weight loss.
It also very much appears to be the case that weight gain and loss are heavily influenced by environmental factors. Skinnier countries aren't skinnier because the people there have more willpower, it seems, but because they live in a skinnier country and are surrounded by the culture, laws, physical layouts of the created world, et c., that come with that. It'd be kinda weird if we expected "just try harder" to work very well when that's evidently not the mechanism by which skinny countries are skinny. Alternatively, if it is willpower doing it, we're just adding a step, because then it appears that environment strongly influences willpower, instead, since the same observations hold.
Sure, sometimes it works for individuals. In fact, it often works temporarily, causing a yo-yo effect. It can work for long periods (many years without a slip) but that's rare.
If your solution to the obesity crisis is "people need to try harder" your solution is demonstrably not helpful. Can it work for one person? Yes. Over a population, will it? No, it won't, it's amazingly ineffective, even very expensive high-touch interventions involving multiple experts aimed at weight loss and lifestyle change and such are wildly less effective than "inject GLP-1 agonists" or "move somewhere skinnier".