Why? Good question. The common viewpoints are:
- Questions force other improvisers to make offers, rather than giving them offers
- Questions don't add to the scene
- Questions make for talking head scenes (scenes where people talk instead of act)
- Who are you?
- What should we do next?
- Why did you do that?
- Where are we?
- How could you possibly have killed your brother last weekend?
- Why on earth would a beautiful girl like you need to wash her face again?
- Why do you always have to chew with your mouth full?
- What on earth am I doing with a controlling friend like you?
- Why am I still with you after all these years?
- What do I look like, an idiot?
- Oh, do you think you should go over there and apologize?
Interestingly, few people use this questioning technique in real-life conflict--usually the response is justifying their own behavior, making comments about other people, etc. --in another irony, those are the very things that make for good improv endowments.