DPRK economic and political development was superior to the ROK which was a cutthroat military dictatorship that was committing massacres left and right. The two Koreas were very distinct.

It's only until the illegal dissolution of the USSR that the DPRK struggled as the US waged a genocidal war on anyone connected to the soviets. The Korean extermination by starvation plan failed so the US pumped a bunch of investment into occupied Korea via the neoliberal transition. This coincided with Bush's axis of evil speech and so NK became the designated enemy.

I've noticed a lot more people saying "Korea" when they mean the ROK which speaks to the success of the West's propaganda blitz. In the US, the Korean war is nicknamed "The forgotten war" even though it was more destructive than WWII.

It might have something to do with the Korean war never officially ending. There's just been a very very long armistice.

my shoot from the hip answer after listening to Season 3 of blowback a few years ago is that the US is the main force against a unified Korea, even conceptually, because it has consistently been the case that the DPRK would emerge as the dominant political project in any "clash" or even completely peaceful and equitable re-integration. the imperialist puppet is a highly dysfunctional organization riddled with gangsters, grifters, property bubbles, and bizarre right-wing cultists who claim supernatural powers. not to mention the various capital formations of chinilpa and their descendants.

without the US presence to bolster the puppet's security apparatus (which was an overt military dictatorship until 1987) and guarantee lines of credit, it would probably fold under the near immediate civil unrest that would follow any left-shift of "acceptable discourse" about worker rights, housing reform, universal education, etc. if you want to read about some fucked up people, look up every "president" of the ROK before the June Struggle of 1987. after isn't exactly great, but prior to that is like a rogue's gallery of the most obvious ghouls and goblins. generals, removed, literal hitler-stans, japan-born occupation collaborators. the Korean War wasn't forgotten in the US for no reason... even the most cursory skimming of what happened then and ever since is generally damning to the US' role and involvement. so usually the focus in the west is on how "kooky" the north is for like... not being in thrall to an industry that makes people surgically and chemically reconfigure their faces and bodies and perform on stage to normalize and induce the practice of cosmetic surgery more broadly in the labor and "matchmaking" market.

According to a Statista survey in 2020 which interviewed a total of 1,500 people, plastic surgery is prevalent among young women in South Korea. Nearly 25% of women aged 19–29 have undergone plastic surgery, while men have only undergone surgery 2% of the time. This number increases to 31% of women and 4% of men among 30–39-year-olds. A case study by Allure Magazine noted that in recent years, the percentage rate of males who underwent plastic surgery increased and consisted as much as 30% of cases of those who underwent the procedure.

but i digress...

the US maintains its relevance by portraying the DPRK as the rogue "other" that its puppet needs protection from and whenever relations between the the north and south warm up, the US ramps up provocations and sanctions or otherwise orchestrates increases in diplomatic tensions, like getting the south korean government to claim control over the entire peninsula in its constitution and require that unification be synonymous with liberalization.

There is a One Korea policy between the DPRK and ROK. Both consider the other illegitimate. As for how they both join the UN, they were essentially given observer status until the 90s when the ROK campaigned to be recognized as a UN state. Since neither China nor the Soviet Union was willing to veto the ROK from joining the UN, the DPRK decided to join the UN as well. This, however, greatly hurts the One Korea policy since the world now recognizes two Koreas.

There is a One Korea policy between the two states. It's just that China, being a part of the G7, one of the strongest members of the UN, and an economic superpower has the ability to throw its weight around economically and politically, while DPRK doesn't have such leverage.