Physical alertness, the sort that you need to shoot a gun or a bow and arrow or other things that require physical activity don't take the same sort of brain power or concentration that learning a heavy subject takes. One has activity built in, the other has seating and a possibly boring environment. That's the point.

It isn't that one doesn't require some level of alertness, but that it is different. You can kill yourself lifting weights or swimming, but folks still do that early in the day - even as they might not be alert enough to sufficiently learn during a boring class.