Hi OP, by using "doesn't", your question sounds like one about current affairs. For your reference, this subreddit does not permit answers about events or conditions within the last 20 years, so if you're looking for information more recent than 1996, this isn't the forum for that. In that case, do consider x-posting to somewhere like /r/geopolitics. However, if you are looking for more historical reasons explaining how security came to be managed as it is, this subreddit is fine.

Because technically speaking the Korean War is not over. The two sides agreed to a cease fire, but no peace treaty has ever been agreed to. Also, any attempt at UN peacekeepers would have likely been vetoed by the Soviets in the Security Council. The Security Council only authorized action in Korea because the Soviets were boycotting meetings at the time. They would have vetoed military action had they not been and they helped North Korea during the war.