I think it was ~1982 that the Atari consoles started spreading, and now there's a multi-billion dollar industry. The overweight/obesity spike began shortly afterwards. A bored kid used to get sent outside to explore, climb trees, build forts, or play some ball... that is, if chores were done. There's dishwashers, roombas, log splitters, snowblowers, riding mowers, and maintenance fees that ease the household chore load for people that used to be the kids jobs and physical training. There isn't the same physical labor load on the average American adult or child.
There were fitness standards when I was schooled, and the PE teacher would show you where you fell among the group and nationally... jumping jacks, sit-ups and push-ups were part of every PE class, then laps/windsprints added. Even stretching is absent from most PE now, let alone ranking compared to others that might shame or embarrass a child.