I’m pretty sure like 80% of North Korean defectors start in China. Obviously, a top diplomat with resources wouldn’t necessarily do that since China wants to keep up appearances (even if they sort of hate North Korea). But “most” “defectors” fare about as well as “illegal immigrants” in the U.S. or Europe.
As cynical as it may be, it’s in South Korea’s, China’s, and North Korea’s interest to ignore most of it. If it gets media attention, maybe South Korea or a Western country accepts them as asylum seekers. A few times, people got into western embassies in Beijing and it was global news. China was not happy. But most people who leave North Korea end up working in a Beijing noodle shop or whatever.
And South Korea, while often proclaiming to want reunification, doesn’t actually want the headaches. It’d probably be 1,000 times more complex than Germany and East Germany reuniting.