"Why don't we teach effective exercise, sports, and fitness in schools?" We do. It's called gym.

Maybe you had better gym classes than me, but mine were tedious and boring and turned me (along with most other "nerds" in my class) off from sport until I reached high school and discovered weight lifting. I always liked Kelly Starrett's motto of "all human beings should be able to perform basic maintenance on themselves" and I wish that point was more emphasized during physical education.