Refinement of the filter-feeding hypothesis: folk who get proper fat have permanently maimed metabolisms due to malnutrition in childhood.
Refresher: "antinutrient" is a weird bureaucratism that means "poison." Every poison is an antinutrient.
Modern children are poisoned with just about every poison, plus their food is decidedly low-nutrition. By inspection, America is a third world country where nobody can get good food to eat, and malnutrition is just a fact of life. (For once, not a joke. America is genuinely a third-world banana republic, it's just the high-cash version. Inflation benefits of reserve currency status, not as dumb as most other places, etc etc.) You need vitamins and minerals to grow up healthy and straight. If you don't get enough, you grow up crooked. Metabolic dysregulation isn't quite as obvious as bone malformation, but, in the end, it's obvious enough.
Having tried weight loss myself: yeah it is not in fact hard.
What you see in the wild is a bunch of studies saying weight loss is functionally impossible, plus some other folk saying, "Yeah just eat less lol." For me, "just eat less" worked immediately.
Conclusion: the studies focus on folk who need to lose weight. They don't take healthy, normal-weight people, make them gain weight, or try to make them lose weight. A) that's work, and we're talking about dieticians. B) it's probably "unethical" or something, because, so say we all, grownups are children. (Hence, I had to and did perform the experiment on myself.) It's not like "eat less" is bizarre occult knowledge. Everyone with a weight problem has already tried it. Everyone with a weight problem is someone whose metabolism is so screwy that "eat less" doesn't work. It is not surprising that fancy diets also don't work on these hard cases.
Probably there's mild and severe versions. Hunter tribes don't get fat at all and have no extra weight to lose. If you have an industrialized (third-world-ized) diet, then you get enough dysregulation to gain weight. If you have childhood dysregulation, it gets baked into the cake, the way a tree grows around a metal pole and if you remove the pole, the hole in the wood remains. It just has a cavity there for the rest of its life.