[snovymgodym]

If you think US foreign policy decisions are, or ever have been, made primarily on the basis of preventing the suffering of civilians in other countries then I have an Aerospace & Defense Sector ETF to sell you.

Snark aside, the real answer is that in the past it was infeasible and undesirable due to the geopolitical situation with China and the USSR. Any major operation there would have been a Cuban missile crisis level provocation. After North Korea successfully built nuclear weapons, any sort of regime change games like the US has done in other countries became totally out of the question. There may have been a brief window in the 1990s where it would have been possible, but it still would likely have resulted in mass civilian casualties (look at where Seoul is on a map).

The only "good" realistic scenario for North Korea in the next 20 years is that they gradually reform their system akin to what China did in the late 70s, early 80s.