The South Park voice actors

South Park is one of the few shows I constantly follow and I've happened to notice a very bothersome issue:
Every guest appearance in the show has the exact same friggen voice.

I didn't notice anything like this in the first seasons, for all I know they still had a crew back then. This cheap production flaw might just have appeared in the last season(s). But how hard is it to hire some different people to voice a character that only makes this one appearance? It's tedious and annoying to hear the same voice again and again, especially when they sound nothing like the celebrity portrayed (like they always address at the beginning of an episode).
South Park shouldn't act as if they're broke; for all we know they are richer than Bill Gates. Get some employment, guys.