First, there's really no need to. They do pose a threat, but they can't really carry out an invasion of South Korea anymore since the South is now both economically and militarily stronger (as opposed to back in 1950 when it was the other way around). The threat of a nuclear attack is always possible, but they're not that stupid. They know if they use nukes, the US is going to turn that country into glass.
Secondly, China probably wouldn't want to deal with the influx of North Korean refugees that would inevitably pour into their country if a shooting war starts up again. They don't seem to like the North Koreans that much either, but they still see it as a useful "deterrent" against South Korean/US influence in the region.
Third, the North Koreans wouldn't be a match against a combined South Korean and US military, despite what you may hear. Unfortunately, they have lots of troops, and lots of artillery, and if they feel they have nothing left to lose (aka their country is being invaded), they'll launch everything they have, and take down as many people with them as possible. The death toll would be in the millions, and all for what? North Korea isn't exactly a bustling metropolis chock full of resources that a conqueror would want anyway.
Fourth, assuming reason 3 somehow took place, I don't think many South Koreans would want to deal with modernizing North Korea, particularly if they caused heavy casualties. Not to mention all the North Korean people they'd have to support and bring up to speed on what's been going on in the world since 1953 (no, Dear Leader didn't create the world).
A peaceful reunification would be preferred, but with both sides' opposing ideologies (the North would want South Korea to fall under their leadership, and vice versa), that probably won't happen either unfortunately.
You can probably see why its such a mess there as a result.