The answer is simple.
China wants the wasteland to be their land buffer between U.S allies and them. It is like that nasty dog that protects the chickens from the fox. The dog barks and bites but it does its job. Plus a fallen NK means millions of unwanted refugees.
South Korea doesn't want their economy to be completely destroyed by reunification. They talk a good talk to appease their population but all the leaders know reunification would devistate them.
U.S knows taking over NK is not going to be easy. It's basically millions of people taught since birth the U.S is the devil. That and it would piss off their Sugar mama(China) and Russia.
So basically everyone is content with NK being what it is. They just hope the dog would shut up and stop being annoying.
The only hope for NK is change within.