I think you're wrong here. The reason why the move was made to submarine launched weapons was its too darn easy to shoot down a missile on launch. It's really really easy to track where these missile launchers are. They are huge big slow wheeled vehicles. The us warships are in the area because they want to launch their ship born interceptors . It's takes an age for the North Korean  launcher to get ready and to prepare, so you just blow it up before it launches. Or if it does launch its trajectory is up, and it's very very easy to work out and predict where it will be 2 mins in flight , by which time you've shot it down.