What enemy would spend munitions on an empty silo, while knowing that the you already shot all your arsenal?
With radar, and infrared satellites everywhere, attacking a second strike weapon makes no sense.
Any attacker in sane mind will fully focus on offensive.
Not saying I agree or disagree with it. But, that's the logic, anyway.
This goes double if nuclear submarines exist (they do). Plus nuclear bunkers are supposed to be hardened against nuclear strike. So they could absorb the hit and still launch. Nuclear weapons are actually pretty poor at destroying protected underground facilities. You need something completely different: a heavy piercing bunker buster bomb full of conventional explosives.
Plus, even if you were magically successful in destroying all of your opponent's nukes without retaliation you've still completely wrecked the environment. Fallout and nuclear winter are still going to ruin your victory.
Like "getting all the missiles already launched by the time enemy missiles arrive" empty?
US has early warning system better than anybody's else, and only 50 spare missiles stored in military bases which will already be taken out in any first strike scenario.