I think it’s important to frame it from the standpoint of actual merit as leadership (what AES states tend to operate on). From this standpoint, I don’t think it’s that different from asking “why do some leaders stay in power so long”. In both cases, the answer could be, “They do the job really well, so why push for someone else?”

Things don’t always pass down well in upbringing for various reasons, but if they do, might as well reap the benefits.

In the capitalist context, we’d tend to call it nepotism, in a derisive way. But the capitalist context is also one where access to “privileged” positions means a better/easier life. In the AES context, positions of leadership can be pretty modest as benefits go and have heavy responsibilities and expectations for devoting yourself to being a public servant.

This is sort of a non-answer of an answer as to the exact cause in Korea’s case, but I wanted to emphasize the importance of why it can matter or not matter, depending on the societal context.

I do remember reading that Kim Il-Sung was very into film and the craft of it (now I am struggling to find where I thought I’d bookmarked a pdf on his study of film). But with that in mind, he may have been more effective than some AES leaders at narrative and passing on a legacy to his children effectively, to the extent that they’d be worthy of filling a high leadership role. That people survive at all to even reach that point also matters. IIRC, a son of Mao died while fighting as a volunteer helping the Korean liberation forces when they were fighting the US forces during the Korean “war” (invasion by the US).