Because it has every right to be that way?

"The number of Korean dead, injured or missing by war's end approached three million, ten percent of the overall population. The majority of those killed were in the North, which had half of the population of the South; although the DPRK does not have official figures, possibly twelve to fifteen percent of the population was killed in the war, a figure close to or surpassing the proportion of Soviet citizens killed in World War II...

Russian accusations of indiscriminate attacks on civilian targets did not register with the Americans at all. But for the North Koreans, living in fear of B-29 attacks for nearly three years, including the possibility of atomic bombs, the American air war left a deep and lasting impression. The DPRK government never forgot the lesson of North Korea's vulnerability to American air attack, and for half a century after the Armistice continued to strengthen anti-aircraft defenses, build underground installations, and eventually develop nuclear weapons to ensure that North Korea would not find itself in such a position again... The war against the United States, more than any other single factor, gave North Koreans a collective sense of anxiety and fear of outside threats that would continue long after the war's end."

• Charles Armstrong, The Destruction and Reconstruction of North Korea