Why doesn't South Korea annex North Korea?

https://www.quora.com/Why-doesnt-South-Korea-annex-North-Korea/answer/%E6%A1%91%E7%94%B0-%E5%81%A5%E5%8F%B8-Kuwata-Kenji?prompt_topic_bio=1

Isn't it a mistake that “North Korea will merge South Korea”?