We all know the problem: processed food and lack of exercise.
We all know the solution: Organic vegetables, lean protein, and lots of exercise.
Maybe? I don't think we all know this. And some research, especially into people with unusual metabolism (but not necessarily known metabolic disorders) indicates that even with these restrictions, they need to operate at a significant calorie deficit simply to maintain a healthy weight. [1]
It might be that they are already exposed to enough processed food and pesticides that simply getting those things out of their diet is not sufficient, but I think it's clear that there's more going on here than a simple answer.
And, I think, most damningly, there are many people who maintain a healthy weight with no active efforts to maintain that weight, including a lack of exercise and consumption of processed food. It might be that a significant fraction of people are resistant to this effect or might just not enjoy the taste of processed food so naturally gain the benefits of avoidance.
Just reducing the vast intake of refined sugar in the US would help a lot.
Processed food isn't the problem. Processed food is the bullet, so to speak.
The person holding the gun is mental issues. Mental issues cause people to eat unhealthily.
We really don't all know these.
There isn't anything even remotely close to a scientific consensus on any of those.
There are plenty of scientists who tell you to stop exercising when you want to lose weight, because the exercise leads you to eat more than you would otherwise, and is ultimately self-defeating -- to focus entirely on eating less, and then only add exercise back in once you've reached you target weight. And "processed food" is a highly non-scientific category that is way, way too disparate to be useful at all.
Organic vegetables might be nice, but there's zero evidence that organic vegetables are better at weight loss than non-organic. Also zero evidence regarding lean protein as opposed to fatty -- there's a big argument that fatty meat is better for weight loss, because the fat satiates so you wind up eating less calories overall.