Why do you believe, or what evidence do you have, for elementary educators spending most of their time keeping children on task? I have asked elementary teachers about this trope and pleasantly surprised to learn that most students do not require constant redirection. Likewise, their classes not much different from higher level classes excepting for the rigor of the subject matter. So, I'd love to read any evidence you have for your statement in case these teachers and their schools were an exception instead of the norm.
Perhaps it can start in high school. I completely agree that there is a developmental caveat here that makes the approach a no-go for younger students.
Most of the time spent is just trying to keep the kids on task not actually spent trying to teach.