Unfortunately the current situation appeases the global powers the most. China and Russia will not stand to have a US allied united Korea so close to Beijing and Vladivostok, which means a united Korea is a neutral, non US aligned Korea, which the US does not want. Having North Korea be a buffer state between China/Russia and the US aligned South Korea is the most stable option, and as a result North Korea knows that it can do whatever it wants and still be propped up by Beijing, just as it has since the Korean war