Because no one is doing it intentionally. So crap happens. Players unintentionally lower heir heads by inches when trying to tackle a guy. Offensive players make last second moves, placing their head in the path of a Defenders head where it wouldn't have before. All kinds of scenarios where the D simply isn't trying to do this. Make it a penalty so you don't have people just giving up on trying. Stop kicking players out for it. NFL is doing fine without booting players.