The supporting casts are similar because for a large part, the production of these shows are formulaic. And they go with what works. Let's say you've come across a new show this season, and the first episode just came out. If say, one sees the person playing the mother for one of the main characters in one of their favourite show, play the mother for the character of a main character in the new show, the little sense of trivia feel endearing to them, and can greatly aide in the popularity of the show.
And a large part of this will eventually lead to typecasting. But with minor actor/actresses, this can be a plus for them. You can thank the really obessive-fanbase niches for making the system work as well as it does. These "saesang" fans really get into the nitty gritty details and enjoy pointing out all the background details and extras (
I'd have to admit that I do as well)
, and the writers and staff really enjoy putting them in/exploit them, however you want to look at it.
But the mass majority of supporting extras will go unnoticed, and that's where the less-than-savoury underbelly of the industry rears it's ugly face.
http://vaultofdoom.wordpress.com/2012/11/23/the-yeouido-observer-006-show-me-the-money/#more-1625
(gotta be honest, found this article off of AKF)