>That is what I came to believe as well. If sugar alone was the problem, vegans would be fat. If fat alone was the problem, ketogenic and carnivore diets wouldn't help people lose weight.

This logic is faulty because both vegans and keto/carnivore people are selected for adherence to diets. If you can stick to either dietary restrictions, you can probably also not pig out on pop tarts or whatever.

It’s the Randall Cycle.

Nobody cares about fat, except as it influences how well they look. They care about good health. There are many people who would be happy to sit in a wheelchair all day (they probably don't know how uncomfortable that is and would demand extra comfort, but the idea). People don't want to exercise (in general). They want to eat good tasting food. They want to drink, smoke, and do other drugs. They want whatever their religion says is good.

Because these are often in conflict they must compromise something. If you find a way to be fat while: looking good, living a long life, and being able to do the other things in life you want through life people would be happy.

Of course being fat correlates strongly with things people don't like about living a long healthy life and so we try to lose weight, but that is only a proxy.