The amount of assumptions in your post are downright comical. I'm overweight, borderline obese, and in my 40s. And I'm very
very aware of how hard it is to lose weight, as I fight the good fight every single damn day. It's taken me the better part of a
year to trim off 20 pounds while consigning myself to a diet of mostly salads and a ton of biking. And I can very easily lose a great deal of those gains with a single cheat weekend or literally any vacation.
Losing weight is hard because the human body is
efficient. You don't have to binge eat to gain weight. Even eating just when your body says "feed me something" is often enough to cause steady gains.
That's the point.
That is why it's hard to keep weight off. Because the struggle literally never stops. Most people diet a bit, lose some weight, and then want to go back to "life as normal", not knowing that "normal" means "weight gain." True weight loss requires
lifestyle changes and an acceptance of a little discomfort, and most people don't want to or can't put up with. How many people do you know that eat a couple of salads as their only caloric intake for the day and bike at least an hour a day? Of the people you know who do that, how many do you know that would be willing to maintain that
for life? Don't go spouting bullshit at me when you don't even know the lifestyles these people you're claiming are trying to lose weight are engaging in.
At the end of the day, people want some semblance of comfort. They want to enjoy a donut, or a piece of cake, or a soda. They don't want to feel hungry all the time or constrain themselves to nuts and berries for months on end. They want to eat a prepared meal instead of a pile of leaves. People don't want to permanently alter their diet to something borderline uncomfortable. That is your answer as to why weight loss isn't sustained. It requires people accepting a bit more misery into their lives permanently. I fucking hate salads. I fucking hate biking. I fucking hate not picking up something to eat immediately when I feel hungry. Yet I've been doing it for the better part of a year now. Because I hate dying more.