There are a few people with genetic mutations that effect the thyroid, which in turn makes their body store almost everything they intake as fat.
A few, less than 1% of the population.
I on the other hand can not gain fat weight, I inherited it from my father. For the last forty years ( since I was 15) I have been 150-155 pounds with a total body fat of 7-8%.
An average healthy, physically fit male is around 12%, and some marathon runners get down to my level when training.
And believe me I eat very well, with desert almost every night. But I try to limit my intake of processed foods which contain a lot of fake food ingredients that you body automatically converts to fat.
Hydrogenated vegetable oils ( which the FDA just banned and will take effect over the next three years). Corn syrup and High fructose corn syrup, which are artificial sweeteners made from corn, they call it natural ,but there is nothing natural about it after all of the chemical processing.
Artificial sweeteners, Aspartame, Sucralose, Stevia, Neotame and a few more that make you gain weight even though they do not contain many calories per serving. A half a can of diet soda makes your pancreas go into overdrive and most of what you consume during that meal will be converted to fat.
So yes there are a very few people with genetic/medical conditions that make them fat ( one of my best friend was always fat, but after thyroid surgery and medication to help control his thyroid he has been a normal sized person for the last ten years) so you can not JUDGE every one by the same standards as you do not know their medical /genetic condition.
But MOST people are fat from over eating or bad diet choice..