Also, North Korea is under the protection of China. Partly because they're allies (sorta), and partly because China just doesn't want people messing around near it. One might recall the last time we tried to take over North Korea. Once we got close enough to the Chinese border to make them uncomfortable, a million or so Chinese troops poured over and made life somewhat difficult for the US and their allies.

And that was 1950s China, fresh out of a massive civil war and struggling to get by. 2016 China is far more powerful.

If you want to complain about how nobody does anything about North Korea, look at who's protecting them to see why that is.