To make matters worse, people generally have *no idea* how calories play into gaining/losing weight. Most recognize 2000 cals as the recommended daily amount for adults since its on every food label, but even that is misleading since it's going to vary drastically based on a person's size/activity, etc.
Like, I didn't learn about how calories impact weight until I finally decided to start losing weight a few years ago, and while trying to figure out what to eat, I learned about how ~3500 cals eaten/burned = 1 lb gained/lost (probably not exact, but good enough for a ballpark estimate for planning purposes). I also had absolutely NO idea how many calories I was eating until I started tracking it obsessively. It is incredibly easy to go over your calorie goal if you aren't watching labels carefully.
Before that, I had no idea and generally thought it had something to do with fat (since for a long time it was commonly accepted that fat makes you fat)
So I can see how people can be totally clueless about why they are fat. There is a LOT of contradicting and misleading information out there regarding weight loss/fitness.