That's definitely true, and it's a bad thing, but I don't think it's the core problem. If it was, you'd see massive differences between cities and the research

That's definitely true, and it's a bad thing, but I don't think it's the core problem. If it was, you'd see massive differences between cities and the research would read like this: "City A is too car-centric, and all the kids are sedentary and obese. City B has great public transportation, lots of parks, and the kids are doing great." Instead, we see the same problem everywhere, including places in Europe and Japan where the outside is just fine.