I mean what do you want to do with NK? Leave it be? Laissez faire? Let it develop nukes as a "deterrent"? Let it be aggressive? This will not end will for the region if something isn't done, and SK is reminded that on a daily basis.
The problem is that nobody knows, and nobody wants to be responsible for figuring it out, implementing it, and paying all the costs. We're all hoping somebody else will fix the problem for us.
The US blames China. China predictably blames the US. South Korea vacillates between being hot and cold towards its Northern brother, yet neither approach has solved the issue. Meanwhile, Russia is all too happy to no longer be associated with the problem in the eyes of the global public, despite its own key role in getting us into the current mess.
You're right that reunification still has majority support in SK, but will that be true in 20 years' time once a unified Korea can no longer remembered by anyone still living?
NK is a complete basket case, with a population that has been brainwashed by 70+ years of misrule, a development level probably below China in 1949, and they have a different language and culture.
Right now, China gets the benefit of a buffer state without the downside of fixing generations of problems. They would take the blame for every problem.
Did you know that the goal of reunification is explicitly written into South Korea's constitution? Yet I've spoken to younger South Koreans -- even they don't really want to reunify with the North, because it would significantly lower the reunified country's standard of living.
It took an entire generation for East Germany to reintegrate with the West, and even now GDP per capita is about 30% lower in the east. And East Germany was only about 20 years economically behind the West. For NK, multiply that problem 10x.