First off, we didn't invade Libya, we supported the rebels there.
As for the question, there are quite a few reasons why we don't go into North Korea:
1) No oil.
2) 1,000,000 land mines along the 38th parallel, which separates the two Koreas. We would lose hundreds, if not thousands, in the initial land attack. This would turn public opinion against the war very quickly.
3) South Korea doesn't want a quick resolution to the conflict. They would rather have a slow handover of power, so as to avoid an economic vacuum which would destroy the economic stability of South Korea as factories and mills moved north of the border. The same thing happened when East and West Germany were reunited.
4) No oil. I know I said that one already, but it bares repeating.
Lived and worked in South Korea for almost three years.