Because they're people too.
If 'we' (I assume you mean the rest of the world) thinks it ok to destroy a whole nation, what's to stop North Korea (if it survives) or anyone else from destroying us?
If any country destroys another, then other countries will take revenge, or say that since so-and-so did it then so can we.
Yes, North Korea makes constant threats in the form of nuclear tests etc. But they're mostly just the government, and as far as I know mostly their dictator. If we destroy North Korea and get rid of this one threat, then we'd also be murdering millions of oppressed Koreans who've done nothing to deserve it, many of whom know next to nothing about the rest of the world due to their ruler not allowing outside communication.
If you still can't work out why killing millions of people is ever a good idea, try listening to this song written during the Cold War - it's about Russia rather than North Korea, but exactly the same ideas.
Resident of Korea weighing in...
Right now, tensions in Korea continue because American military contractors are generous campaign donors. That won't always be the case.
All Koreans expect an eventual reunification, one that will be made more difficult and expensive by a huge land war. Americans want to scale back an expensive and open-ended military commitment. China wants a stable, prosperous neighbor that will continue to trade with it. Russia has a tiny land frontier with North Korea and would like it to be more stable.
All involved parties are ready for a war, but no one will really benefit from one.
Because the balance of powers will be tilted severely once the North Korea's country or government is destroyed which will definitely cause WW3.
China and Russia have both invested time and a lot of resources into North Korea to keep the US-occupied South Korea in check. They even invaded the South at one point.
Once the NK government and/or country is destroyed, the remaining North Koreans will flock to China and the South will reclaim the land of the North making it under US control which completes the Pacific Belt program of the US where China and Russia will surrounded by India, South East Asia and North East Asia.
The Chinese and Russians will realize this and thus they will mobilize their military to reclaim the North Korea's territory just like what they did when the Americans crossed the Yalu river.
All hell will break loose if the North Koreans are attacked. The Chinese and Russians don't like them but they are vital in balancing the political leverage the Americans have.
Besides, NK is a dog that constantly barks but still hasn't bitten anyone yet. Kim is the only one making threats to secure his authority over the country. Should he ever decide to attack, all his kingdom that his father and grandfather built will be all for nothing.