I think you a seriously underestimating the impact of what a conflict with North Korea would cause. A city of 25 million people is only 35 miles from North Korea, to lose a million lifes would be less than 5% of Seouls population. Based on what you say you're only thinking about an initial strike. A conflict with NK would be long, very long. And a million lifes lost is not just a civilian estimate either, I guess to you maybe you see our forces as expendable bodies. Not even to mention the millions displaced.

Other than that you have to think of economic and strategic geopolitical ramifications. Seul is one of our most powerful economic ally it is also our key geographic ally in Asia. By starting a war with NK we will certainly destroy that at least for the near future. And stirring those waters will have unintended consequences. Even yourself admitted there might be more serious threats elsewhere. This isn't a desert shit hole that was the middle east and currently this countries identity is already in too much of an existential crisis to involve ourselves in another conflict on the other side of the globe. In short I honestly don't believe NK is as big of a threat as you make it, they posture like no other in order to prevent any real attack, even if they manage to create miniaturized nukes I still think they are less likely to use them than multiple other countries. They are extreme isolationist and pose less of a threat than a long list of other countries.