For children is much easier to learn how to run close to the same, predetermined opposing offence player then how to independently assess which player to defend depending on the situation of the game, ie. it is easier to learn how to put pressure on one offence player than how to read the game.
It is easier to learn how to catch the offence player, hang up on him or wrestle than how to do correct handball foul or how to steal the ball without the foul. First described we like to call pressing, and second described irresponsibly playing defense.
Kids under pressure of the coach will play defense effectively, but this way of learning is not sufficiently effective.
It is sadly true, but the kids will win more matches under pressure and some coach will use it for selfish goals. To promote himself.