South Korea would have to be nuts to try to absorb the tottering wreck that is the North. China has no real interest for the same reason, and also because Korea has always been ethnically distinct from China while still a vassal state of the Middle Kingdom. As a buffer state, North Korea is ideal, as long as the regime can keep its hair on.

China may instigate a regime change if forced, but they will use Koreans to do it. Most of what passes for the Korean middle and professional classes have fled to China over the years, through the North's more porous and less dangerous frontier. The Chinese would prefer to set up a nice, compliant little Korean buffer state run on the Chinese Communist model. They have a whole cadre of people to do that, but they would prefer, imo, to wait for the regime to implode rather than to start something they then would have to be responsible for cleaning up.

The North knows this. Why do you think they want to attract our attention? Who is the logical counterbalance for China in that part of the world? South Korea? Laughable, besides which the North has nothing but contempt for the South. Japan? No real army, even though a formidable economic force. Russia? Has enough problems on its own southern border, and has terrorists of its own. Who other than the US?

The North has no way acceptable to themselves to get us involved by friendly behavior, and they have nothing we want. So, like a kid who wants attention, they did something we don't like in the hope that we will talk to them to prevent them from doing it again. It's the only hand they have to play; they may have overplayed it this time.