Well, one thing is true, if an evil genius wanted to make people sedentary, inventing classrooms and teachers who frown upon fidgeting would be an excellent way to :)

A related issue is that at some point around puberty kids find it no longer matching their half-adult dignity to just run around playing. At that point they don't really have many ideas what other ways to move.

I think it also got less social, people used to do sports together, now run or lift alone.