The number one reason we don’t shoot down North Korea’s missiles is that we cannot…
If North Korea cooperated and shot their new intercontinental ballistic missile, the Hwasong-14, at the United States with adequate warning so that we could prepare, and if the warhead looked pretty much like we expect it to look, and if they only shot one, and if they did not try to spoof the defense with decoys that looked like the warhead, or block the defense with low-power jammers, or hide the warhead in a cloud of chaff, or blind the defense by attacking the vulnerable radars, then, maybe this is true. The United States might have a 50-50 chance of hitting such a missile. If we had time to fire four or five interceptors, then the odds could go up.
But North Korea is unlikely to cooperate. It will do everything possible to suppress the defenses. The 1999 National Intelligence Estimate of the Ballistic Threat to the United States noted that any country capable of testing a long-range ballistic missile would “rely initially on readily available technology – including separating RVs [reentry vehicles], spin-stabilized RVs, RV reorientation, radar absorbing material, booster fragmentation, low-power jammers, chaff, and simple (balloon) decoys – to develop penetration aids and countermeasures.”
Our anti-missile systems have never been realistically tested against any of these simple countermeasures.