Why do unannounced games keep “leaking” through Korea’s rating site? It turns out there’s a legal issue that makes it almost impossible to avoid

They could avoid it, if they released the games at a later time than the rest of the world. But I do not think that its worth it.

Why are games being submitted for ratings before they've even been announced? I thought that was something that happened very late in the process...

If you think you’re on the borderline between the local equivalents of M and T, and are willing to revise your content to get a more marketable rating, maybe you submit early? I know that’s a thing for movies in the US, as PG-13 movies can attract a broader audience than R. If the ratings board isn’t looking at the final product, though, can they rate it correctly?

Censoring certain content based on country is super common in video games, so early submission may give more time to appeal or censor stuff to meet their desired rating.
Or just market to a different rating if they'd prefer.