Because they're funny on many levels.
Billy Braggs too much tell him to stop it!
You can't! You don't know which witch is which!
You can mash potatoes, but you can't pee soup. (sorry sorry. Really. I've loved this joke since I was... oh.. six...)
Because they like being amoosed!
An elevator. It lifts the spirits.
Being stuck in an elevator with the Doublemint twins.
They work on so many levels.
BUZZ LIGHTYEAR: I have an idea...