Earlier quoted context omitted.
I think one of the major reasons no one does anything about it, ignoring the buffer China enjoys from the US (via proxy), is the humanitarian crisis from the entire NK population after the war is over. Who is going to help them? China? SK?
South Korea already has plans in place for that. I can't find the article at the moment, but they feel super bad for and about North Korea and really want to help if they could
Not a very literate and informed reply I'm having here, but just from friendly conversations with a few South Koreans, what I'm getting is that the current generation, many of which haven't grown up in an active war situation, have a much smaller appetite for reunification than their elders may have had. And while they have plans in place (there's a Ministry of Unification) I doubt it that South Korea has money set aside for the real cost of unification (since those amounts would be so tremendous that it'd probably be economically and politically irresponsible to withdraw that much money from the active economy for this long). But then, I'm as little an economic expert as I am an expert on Korea's reunification...