Come on. The actors don't know how it is to do the voice in a cartoon and just read up their texts. They probably read it for the first time when they make that show.
You clearly have no idea who they are, do you? Have you ever even
watched the show? The actors are so good, that when it comes to dubbing, the other language producers need far more actors than the English use regularly.
The main cast includes Billy West (Fry, Prof Farnsworth, Dr Zoidberg, Zapp Brannigan,
misc.), John DiMaggio (Bender,
misc.), Katey Sagal (Leela), Maurice LaMarche (Kif, Don Bot, Clamps,
misc.), Lauren Tom (Amy,
misc.) and Phil LaMarr (Hermes,
misc.). Of course, there are guest actors and such, but in generally, most of the voices are performed by this array of actors.
You can look each actor up on
IMDb, and see they are highly quality people. The fact that Billy West can manage to produce so many completely different voices simply amazes me. Shock and awe, brother.