Because Vietnam, Iraq and Afghanistan are not what you call conventional warfare. When an enemy has no clear uniform, use guerrilla tactics, blends with civilians and uses innocents as diversions or shielding it's a completely different battle. If NK tried to actually all out assault another country you'd be ignorant to not realise a couple dozen other countries with a combined might unmatched would be all over them. No question. Although logistical issues won't be as bad since they're a bordering country (NK/SK).
Unless you're out to actually invade, annihilate, take and own another country which is far fetched today, when you (rightly) care about civilians it creates issue for military effectiveness. When it's a straight out slugfest it's a different story. Let's both not assume without the details, none of us really know in the end but if you don't give a damn about collateral loss of life and damage then it's gets very, very nasty.
But I will say you're a little ignorant to warfare tactics and history, fighting an enemy you can't tell from friend or foe is incredibly hard, especially from a force who by all rights give a damn about civilians and the enemy do not.
If every enemy in middle east battlespaces wore a uniform it'd be so very different and that's why the last incursion to the middle-east in the 90s was, a relative cake walk (do not read into this as me saying it was easy) when up against a clear enemy. Then of course there's issue like highway 80.