It's the whole society. Despite all the hype out there about terrorists, and unsafe technology, we live in a very safe society, people can be fat and live longer than healthy fit people could 200 years ago. There's very little encouraging people to stay active and fit. Despite all the weight loss commercials, there are just as many fast food commercials. It's easier to be fat, than thin.
Because of this, even thin parents can actually be allowing their kid to be overweight, just by not doing enough with them, or not allowing them to do enough.
Sports help, but not everyone is interested in competing like that.
I used to be overweight myself, I have never been skinny. Right now I'm very muscular and strong, but at my heaviest I was near 300 pounds at 15.
I was never obese, because I was always tall, and had natural muscularity just from doing odd sports and hiking. But the reason for my being overweight was that was I never had a reason not to be overweight. The food was there. I was always considered giant-like, which was a great excuse to weigh a lot, it made people awed at me.
I don't really blame my parents, cause they both worked full time. But since I got into Water Polo, I have lost a huge amount of weight.
The problem is, when kids are young, they don't care that they are overweight, they are fine being overweight. The problem is when they get older. So if a parent doesn't do anything about it early, the kid will most likely just get obese eventually. And once someone is heavy like that, it's harder to get them to do anything.