"Just stop being depressed and enjoy life".

People are overweight because something in their diet, psychology or physiology overrides the natural desire to stop eating when they've had enough calories. As long as that thing isn't addressed, trying to use willpower to overcome it can easily lead to burnout and disappointment. Sometimes raw willpower works, but most people who are overweight have tried that and found it doesn't work for them.

Those people aren't failures, they just happened to have a problem that didn't eliminate itself when using willpower. If your problem is the chemicals in fast food, then stopping fast food by willpower can solve things, but if your problem is pica for some vitamin deficiency than stopping fast food by willpower will not solve things. People that stop by willpower alone are lucky, nothing more.

So most people who are overweight do in fact need more scientific knowledge, or better environments, or both. A pedestrian-murdering hellscape isn't great for getting enough exercise. Micronutrients, letting your stomach rest, avoiding blood sugar spikes and dips, metabolism-affecting drugs like caffeine, stress eating, etc can all affect things.

And because people can't just get up and move to a pedestrian-friendly area, or because vegetables are twice as expensive as meat per calorie, or because their job requires them to sit still for eight hours, they want to try the messy imperfect solutions that do as much as possible in their limited environment.

I can well believe that intermittent fasting works better than "burn more eat less" for someone with the unnatural lifestyle of sitting in an office chair for hours straight. The traditional 3 meal structure was built on a society where people did lots of physical labor throughout the day every day, so just trying to eat less in those 3 meals doesn't change the fact that your body needs far fewer calories at certain times than that diet frees up, and the same goes for exercising outside of work hours.