It does happen, and it's disingenuous to say it doesn't. And not always due to issues of one sort or another.
One of my Aunts had 4 morbidly obese children, who are now morbidly obese adults. All of my father's siblings are at least obese, if not morbidly so. I was 16 stone 2 years ago - obese. Two of my kids obviously have the same genes for being fat, as it doesn't take much to make them look chubby. (The other one is a beanpole.) So there is definitely a tendency to gain weight easily and hold onto it. That's a reason, but not an excuse, because we know that.
And actually, having radically changed my eating habits, I don't gain weight "easily" any more. It was masses of refined carbs, something to eat every time you have a cup of tea, huge portions etc. I now find that I can't actually eat a large meal in the likes of McDonalds - two of my brothers will eat that plus more without thinking about it, easily 1000 calories in a single meal.
One of DD's friends is morbidly obese. 8 stone at 8 years old. She's much taller than the others, too - because overfed children grow up as well as out. I think that is very often missed, people whose children are fat say "oh, but he's so tall," that's often part of the same issue. DD's friend can't run, can't bend to tie laces, can't dance, can't wear clothes designed for her age group - it's a miserable life. She also has signs of early puberty, such as body odor. All caused directly by her (also obese) parents' behaviour. It's very, very sad to witness.
But apart from this wee girl, at least half of the girls in my DD's class are big enough for it to be quite worrying - but of course, our perceptions are so skewed now. We think it's normal for kids to be fat. Think back to the "fat kid" in your class - I looked at a picture from school recently, the girl seen by everyone as overweight was actually smaller than a lot of kids now.
So maybe OP's snapshot was unfair, maybe this family are tackling the issue, but it is downright cruel to allow children to become so very fat. It sets them up for a lifetime of difficulty with food. And it absolutely does happen.