One reason they can’t sit still is because they never play outside. They don’t spend time running, climbing trees, riding their bikes, playing games they organize themselves. In other words, they have a lot of pent up energy that never gets spent.
One of the differences is that when I was in school, our teacher supervised us for everything. There were no “aides” that came in to help. The teacher was always watching — she even ate lunch with us. So, there was no pulling the wool over her eyes. And there was plenty of opportunity to run off steam and plenty of opportunity to organize our own games — something that little league, etc. does not allow.
On rainy days (which were few and far between in CA) we had Folk Dancing.
When we got to 7th grade and above, we had an hour of PE daily. I don’t think the kids get that much any more. I hated PE — except for dance == because I never made good grades. But, it was necessary.