I'm not expert but I think the answer is simple...
When I was a kid (born in 1949) we rode bikes everywhere and constantly. If we weren't biking we were walking...mom or dad only drove us somewhere if it was more than a mile or a bad storm; a little rain didn't qualify.
We'd play tag, hide n' seek, 1-2-3 red light, dodge ball and all kind of made-up, running, physical games until dark.
We played baseball, basketball, football...you name it....year round...even if out in a field or in the street, even if not Little League, Pop Warner or school-sponsored.
In the summer we went swimming and hiked in the woods.
I used a push-mower on a 1/2 acre of grass, no riding mower / lawn tractor.
If you ate a cheeseburger, drank a soft drink and had an ice cream you'd "burn" it off.
Today the kids are either on a game machine, PC, tablet or phone if they're not sleeping or eating...no chance to 'burn" off any of those same foods we ate as kids.
Even out here in a rural area the kids rarely ride bikes or walk...it's 4-wheelers (quads, ATV's, ORV's, etc.). Most kids today drink 4 or more drinks a day sweetened with high-fructose corn syrup as do their parents.
You do that day-in-day-out and continually sit on your butt and guess what......your gut and butt is gonna get big....it's really that simple, IMO.