Usually because they are not good to man to man defensive teams. Good man to man teams take it as a challenge to defend out of bounds plays underneath their opponents basket.
You have got to slide thru screens on all out of bounds plays…not switch. The defensive man whose offensive man is setting the screen away from the ball must open up and pull his teammate thru the screeen.
If defensive players switch on these screens away from the ball it is easier to free up the defender being screen, plus having the man who set the screen come back to the ball is a very effective move.
The defensive man whose offensive player is setting the screen away from the ball is responsible for letting his teammate being screened to know that the screen is coming and then to open up and pull his teammate below the screen so that he/she can still keep his man on defense without having to call for a switch.