I'm not disagreeing with everything you say.  In fact, I agree with most of it.

Russia and China has leaders that seem relatively reasonable, unlike NK in Kim Jong Un.  Additionally, Russia and China has valuable assets, like historical cities and sites, prosperous economies, etc.

NK has nothing of value.  Satellite photos at night show a blacked out country. NK can't even feed it's people.  Thus it's a country with nothing to lose and everything to gain by acting the way it does.