Why hasn't North Korea attacked?

Since the 70’s North Korean offensives against South Korea have only been at the negotiation table. North Korea has been careful to be only belligerent enough to give credence to their bluster at the negotiation table but not to such a degree that would provoke a substantial response from the South. What explains this inaction? Snyder seems to be arguing that North Korea has made a conscious decision that the best way to extract concessions is through a series of hard nosed and unpredictable negotiation stances which has obviated the need to actually invade. Oberdorfer makes the case that the various parts of the North Korean government do not have know what the strategy actually is, so they cannot present coherent policy positions.  

The combination of these two observations probably explains why it is North Korea has never actually attempted to forcibly reunite the country. Kim and his son have nurtured a personality cult but there is evidence that they must nonetheless still keep factions within their government off balance to prevent the rise of alternate power bases. The different style and tactics in North Korean negotiation are not so much Kim’s grand design, but instead are the concessions he must offer to the various factions who are vying for preeminence. The fact that the North has not invaded could be a rational assessment of the dominance of South Korean and American combat power but is probably because communist states cannot take precipitous action without unanimous consent among the ruling clique.