Because China finds them amusing as we might find a mentally retarded merman amusing, and doing something against them might piss off China, which since they have 'Murica's nuts in their pocket, they can't afford to do.
That's what it was last time I checked anyway.
Not quite. China doesn't want to share a land border with the U.S. Army. South Korea is still in (and recently renewed, with substantial support) an arrangement where the U.S. has overall strategic command of both U.S. and South Korean forces on the peninsula, so any scenario where the North Korean regime collapsed, was invaded, or willingly accepted reunification would be unacceptable to the PRC. Theoretically China would be favorable to Korean reunification if the U.S. completely withdrew from the peninsula, but that's both unlikely given the current goals of U.S. foreign policy re: China, and even if it weren't, Chinese officials would have no reason to believe U.S. claims. The PRC tolerates the DPRK and maintains their relationship in part because they're a useful buffer zone, and in part because it's generally wise to keep the unstable nuclear power on your border pointed at other people.
Incidentally, the argument that the U.S. can't do anything too aggressive against China or Chinese interests because of the portion of the U.S. national debt they hold... sort of ignores the fact that a substantial amount of their trade is still with the U.S., and collapsing the U.S. economy (ignoring whether that would actually be possible with their current assets) would cause substantial harm to them. That's fundamentally unacceptable to the PRC; the literal foundation of their constitution is to further the economic development of China, which is why the vast majority of their efforts since '89 have been focused on exactly two things: economic development and the pursuit of the One-China policy, which have often gone hand-in-hand (see their relationships in Asia and sub-Saharan Africa).
edit for clarity: That's not to suggest that it wouldn't theoretically be possible in the future, if they managed to sufficiently diversify their export partners (which they
are doing), but even then they only hold ~20% of U.S. Treasury securities, and the premise relies on a scenario in which China is willing to accept the sort of international reaction (both political and economic) which would result from such an action, and which could easily harm them at both second- and third-hand, leaving aside actual responses directed at the PRC. Or more simply, I don't buy the China Threat argument on the military side of things, and I don't buy the idea that they would leverage that economic power in such a destructive and provocative manner, for a number of different reasons, the most important of which being that they don't
need to upset the international order when it's working in their favor.