I'm going to copy and paste this:
So inevitably, the question that always gets asked is - why isn't the world doing more about North Korea?
Any war, not started by North Korea, is simply a lot of risk with too little reward
As callous as it may seem, with 25 million oppressed and often starving citizens, the risk to nations that would be willing to topple the Kim regime is simply too great. Even ignoring the political issues around doing such a thing (whether the UN, particularly China, would allow it), consider that:
The US has 28,500 troops in South Korea, and over 30,000 more in neighboring Japan and Okinawa, who are the first troops to respond to any war in Korea. The US has been there for over 60 years now, and will be drawn in.
South Korea has hundreds of thousands of troops, with hundreds of thousands more in reserve. Many are along the DMZ, where if hostilities flare, they will likely be caught off guard.
While Seoul is close to the DMZ, it isn't as close to currently established North Korean gun emplacements on the other side of the DMZ. That said, there are parts within reach of longer guns, and as the Yeongpyeong Island bombardments show, civilians are easily caught in the fire in a artillery exchange.
The Korean War cost 2-3 million lives over the course of 3 years. While North Korea can't win against a war against the South + US, even a short conflict will be very bloody. Both sides have spent 60 years planning for one another, and have positions marked and weapons in place.
Even aside from the costs of the direct conflict, the humanitarian cost will be huge. 25 million citizens of North Korea with little in the way of valuable worldly education - many have been indoctrinated into the Kim cult at an early age - will trigger a disaster with refugees pouring into China and the South.
Keep in mind the heavy cost of German reunification, which was peaceful, and one that dealt with far smaller of a difference in culture and mindsets. Or the costs of integrating Eastern Europe with Western Europe after the Iron Curtain fell and a lot of the social costs that have come of it.
Geopolitics have shaped how the world deals with North Korea
Throughout the Cold War, with China and the Soviet Union as its benefactors, North Korea was largely shielded despite being the aggressor of the Korean War, a war that only ended in a cease fire, not a formal end.
At the end of the Cold War, many predicted North Korea would either collapse on its own or reform as China and Vietnam did. Instead, North Korea held on tight - Kim Jong Il even being more repressive than his father, while doing little as famine killed an estimated 2-3 million in the North.
Through it all, China has helped prop up the regime - again, in part to avoid the possibility of a flood of refugees across its border. In recent years, China has started to care less about North Korea, and even joined in on the condemnations and sanctions. Still, they are the #1 business partner, and strategic considerations (like not having a pro-US nation directly on its border) means they tacitly accept North Korea.
And the question always arises is what would happen if a war did erupt?
Would China join in, as they did in 1950? They've never officially left as a party to the cease fire, and have never stated their position. Its highly unlikely they would, but is that a risk the US and South Korea are willing to take, even if the human costs are ignored? No.
Staying the course while waiting for North Korea to collapse has kept a relative peace, but it may not see an end to North Korea anytime soon either
For the most part, all the parties are generally okay with the status quo, given the costs. That said, the hope has long been that economic sanctions and political isolation will eventually cause the Kim dynasty to collapse from public revolt, or from a coup near the top.
Even then, the Kim family has outmaneuvered all of that. Dissent is utterly snuffed out, and their political prison camps are notorious for their brutality. And a coup at the high end? Kim Jong Un had his own uncle executed. In fact, according to analysts, of the hundreds of Kim Jong Un's father's generals and commanders in the photo taken with him upon his accession to the top, over half have been purged/executed.
As the saying goes... any regime willing to kill as many of its own people, tends to stay in power a very long time