I'm going to relapse into geezerhood and say the reason Americans are so far is they use strollers for their children. It's a good way to containerize and control your kids, but it doesn't get them used to exercising.
The other day coming back from the garden I encountered a woman pushing a 2x stroller with a couple kids sprawling across the seats. Granted, if the three of them were walking, the woman would have had problems; the kids looked as if they'd be two handfuls.
I was reading something somewhere the musings of a person who observed really young children in a foreign society being useful and handling dangerous tools, like a machete at age 3. Now that's a tad young. If I remember I was kept out of the barn until I was 5 or so, parents thought it was too dangerous. But I was anxious to help, to prove I was old enough to do something. That's not something usually possible for today's suburban kids.