They teach it because it is productive. Its team productive at the high school level. You dont recruit in hs. It is so much easier to teach everyone the same thing than to teach each hitter as an individual. So they clone them. They teach them to all chop wood. Its productive at the high school level because it limits fly balls. Fly balls are easy outs. It produces tons of fielding opportunities. And at the hs level some will get through. Some will get booted and some will be thrown away. So they teach it because they believe it gives them the best chance to win with the talent they have.
What it does is allows the poor hitter to put the ball in play on a more frequent basis. It also destroys the hitter with potential to be a very good hitter. Since more hitters fall into the first category - there you go. Its kind of like the youth coach that says "Hold the ball and run it back it." Yes that will allow you to win more games because you wont throw it away and allow runs to score. But it kills the kids ability to actually learn how to throw and catch in a game situation.
I am not trying to bash hs coaches. All of them do not teach this. But many do. I have seen it first hand. And I have heard this philosophy preached at clinics in the past. And it made me sick to my stomach. And the previous poster is 100% correct. The longer you wait the harder it is to change. And is some cases it can never be totally reversed once truly engrained in the hitter.