Arms Economy S. Korea 2011
That’s because it has a chance to wreck their economy. Well, that’s at least one of the reasons. They also don’t want to have to kill their fellow Koreans if they don’t have to.
They probably observed the reunification of Germany with interest. Their reunification would require far more support (i.e. South’s money & resources). I think for the moment they’d be fine if the North idled on without too much mishap.