Maya's and Tigers and Deans, Oh My - S5-E15
Trivia: In this episode, Zigmund, who is dating Nina Van Horn, is a flamboyant magician who works with tigers in his magic act. This is a spoof of the magicians Siegfried and Roy who worked with white tigers in their act. At the end of the episode, which aired in 2001, it's suggested Zigmund lost focus and his tiger attacked him. 2 years later, in 2003, Roy Horn would be attacked by one of his tigers.
Answer: Because most voice actors voice a very wide variety of characters, Hank Azaria voices 18 recurring characters and Harry Shearer 15. This is in addition to whatever minor or one-shot characters they may be voicing. It would make the credits either too long or too cluttered on the screen.
LorgSkyegon