Some part indoctrination with some part apathy, wrapped in self-aggrandisement.

People are raised into the propaganda obviously, and that plays a role but I think it is exacerbated by the fact that people often have no real desire to look into if it's true or not. That is the more insidious element. Anyone who actually put two minutes of thought into the claim that Koreans push trains to work everyday would realise it can't possibly be true, but they don't put those two minutes in. If they think about it at all, they just go "Oh, North Korea is so crazy, look at them. lol". They'd much rather have a crazy mystery country to meme about AND to consider themselves better than. They get to feel superior while pretending to be empathetic to an oppressed people.

A lot of liberals are very eager to feel superior to a marginalized people while simultaneously pretending like they care. Propaganda that feeds into that is unlikely to be questioned because they actively want it to be true.