For instance, going with the first strike plan to take out North Korea before they attack, that guarantees that tens of thousands, maybe more, civilians will die on both sides, and as many or more military personnel on both sides.
Tens of thousands would be a basically bloodless invasion. In reality, you'd be looking at losses in the hundreds of thousands on each side, for both civilians and soldiers. And that's being optimistic. Long term, likely well over a million dead, with ten times that injured, displaced, left homeless, starving, etc.
Declaring war on North Korea will result in far more death and destruction compared to one of their bombs going off in even a fairly populous area. Of course, this way it's them dying and if they bombed us we would be forced to invade to prevent it from happening again regardless, but it is not something to be done lightly to say the very least.
Perhaps, if we could get the South Korea, China, and preferably Japan and the USA to all agree on a plan to deal with North Korea once and for all, then the invasion would be a good idea (though still very bloody). Without that, you have to worry not just about the immediate war but knock-on effects as tensions rise in the area possibly sparking further conflict.
In reality, probably the best case scenario is one where NK doesn't collapse at all, but instead softens into a more modern state that actually serves its people. Becoming more democratic and/or capitalist would be nice, but if they instead changed to look more like China does these days, with Kim still at the head rather than the communist party, that would still be a very good end result, at least in the USA's eyes. Even South Korea might be OK with that, since it would be a step toward reunification, albeit a very small step on a very long road.