It does far more service as a buffer state, and the Chinese aren't stuck with the costs of administering and rebuilding it.

If I were in the Chinese government, I wouldn't want to have to "adopt" the North Korean million-man army, and I wouldn't want a border where my border guards were facing off against American GIs.

The Chinese have become very conservative, as Communists go, very interested in slow change and stability. Would a nation pursuing change in a "one step forward, two steps back" pattern want to experiment with regime change in such a volatile place? Oh, I think not.

But still, how much you wanna bet they have a cadre of North Korean refugees that they have been training, just in case the current regime falls precipitously, to put a nice, slow, China-friendly government into place, so they can keep their buffer?