A big part is how the parents raised their kids from early on. Especially prominent in the west. Kid cries, kid gets sugar, kid stops crying, kid knows "when I complain I get the literally most dangerous legal drug". Kid repeats. Parents repeat. Kid gets fatter. Kid wants more sugar. Boom, death spiral of obesity. By the time the kids can make their own choices , they're already addicted, undereducated on nutrition, and probably got body issues and mental health baggage stacked higher than a McDonald's breakfast platter...
Let's not forget that it isn't hard to become fat at all with not much effort. There's no need to cook your meals anymore. Fatso's just ride over to McDonald's, stuff down 2 big macs and throw the paper bag away. There is no such thing as meal prep for fattards.