North Korea knows that the US knows that if the US invades or attempts a "decapitation strike" there's a very good chance they'll get off at least one nuke and that nuke has a very good chance of hitting a very vulnerable target.
By having their hand on the button at all times it means the US can't pressure them militarily, and if the US can't do that, the US can't really do anything.
Is the US prepared to exchange millions of lives in exchange for bombing North Korea into oblivion and, if Iraq is any example, facing decades of anti-US terrorism? It's not something you can win with military force at this point.
The only way to fix North Korea is to try and carrot-and-stick your way out of it.
By having their hand on the button at all times it means the US can't pressure them militarily, and if the US can't do that, the US can't really do anything.
Is the US prepared to exchange millions of lives in exchange for bombing North Korea into oblivion and, if Iraq is any example, facing decades of anti-US terrorism? It's not something you can win with military force at this point.
The only way to fix North Korea is to try and carrot-and-stick your way out of it.