Continued bribes are far less costly than a war only maniacs want. Any war would be quite short, especially if it had China's full blessing (not to mention Japan and Russia's), but it would likely cost tens of millions of Korean lives alone unless the dead man's switch can be deactivated somehow.
Not to mention that the entirely peaceful re-unification of Germany had tremendous costs that weren't anticipated even with one side bending over backwards to absorb them and those parts weren't as far apart developmentally as the divided Koreas are.
The main point of the six-party talks with North Korea have always been to attempt to manipulate its decision making by letting it play the powers against each other in superficial ways. The best example being when we got "mad" at South Korea for holding its own talks with North Korea without our prior approval...something everybody in the region had wanted for years but only by the play-acting was North Korea finally roped in and agreed to things. That China, Japan and Russia also got to let off a bit of steam with each other in the process was the gravy. The five powers are far more in alignment than we pretend because otherwise we don't get that sixth party to do anything but pull back and shoot at stuff. (We essentially did the same thing, in two stages and with different parties, to certain factions within Iran much to the consternation of Johm McCain and neoconservatives.)
Of course, it often comes at odd times when the "world" finally decides to stop the easy route and just break it. Often for no apparent reason. Both Saddam and Ghaddafi twice found out how quickly promises aren't worth anything. How many lives has Basair al-Assad had since taking office? There are factions within North Korea that have to know that China's limited interest in the continued existence of it as a state are only out of China's short term interests which they would gladly waive with proper American assurances.
The main problem is that those assurances can't be made without tipping the hand that kills tens of millions of Koreans.
edit: what North Korea's been incredibly good at and the reason they even probably still exist as they do, is being flat out amazing at continuing to maintain a single facade for decades on end
iirc, there was some hope when the Soviet archives were opened that there'd be a strategy guide in there, but nope, they held the same single face to their benefactors, all the basics are useless, defectors almost never bring out useful information, etc.
