Earlier quoted context omitted.
A friend that is a doctor specialized in metabolic problems told me that around 5% of the cases of obesity have some medical cause, not eating too much. 5% is low, but it is a lot more than "very few exceptions". (Disclaimer: the number is for Europe, I have no idea what is in US)
All obesity is fundamentally caused by eating too much because fat contains energy that HAS to come from food.
It’s true that the extra stored fat might must come from calories not used or excreted elsewhere, but that might be because their body is very poor at making immediate use of the calories as energy or is unusually efficient at storing energy that might otherwise be used. To put a number to it, a 2000 calorie diet as normally recommended may leave them depleted of energy and increasingly fat.
It can take years to discover that kind of “outside the norm” issue, and obesity can easily set in before its recognized let alone addressed. And of course, once obesity does become a part of someone’s life, there’s a whole spiral of challenges that make it hard to overcome. Even moreso when your body doesn’t work normally.