One of the big problems has long been that North-Korea has a large network of artillery within shelling distance of Seoul. Now they also have rockets and nukes and stuff, but even without those any serious threat to NK would come with devastating cost of civilian lives in South-Korea.
See e.g. [1], which estimates that "the DPRK could cause thousands of casualties in just a minute and more than 100,000 in an hour" and some of the case studies in that report have more than 200,000 in an hour. And all of that assumes only conventional HE rounds – no chemicals or nukes. And all of that is in the first hour alone: a full war would cost millions of lives (during the Korean war, South Korean casualties were about a million civilians and 250,000 soldiers).
You need to move carefully when you've got almost 6,000 pieces of artillery pointed at your head, never mind nukes, and the people in control of them seem rather unhinged.
I've seen people express this kind of "we should have just intervened" opinion before, and in principle I don't disagree, but it's really not that simple: both due to the strong risk of heavy civilian casualties, and the geopolitical impact with the USSR during the cold war (and to lesser degree, contemporary China and Russia – I can already see China invade Taiwan using "but you invaded NK!" as an excuse).
[1]: https://www.rand.org/pubs/research_reports/RRA619-1.html