Harut jan, asking a question has always had many meanings, don't u know ?
People ask questions for many reasons. One of the reasons is to know the ideas/opinions of the person they have asked, on that subject. If many people ask us questions, it means not only that we might know things other don't know and want to learn from us, but it also means that they just want to learn more about US, see our opinions, compare with theirs... just to get to know us better.
Besides, there are always better answers than just a "DUH". Infact, the "answer" DUH has been the cause of many bad things in the world: Q: "WHY ARE WOMEN MISSTREATED AND SUPPRESSED?". A: "cause they are women, DUH!". Q:"WHY TO SLAVE BLACKS?". A:"cause they are blacks" and so on. the history is full of examples when horrible things happened as people were too lazy to come up with a REAL cause and reason of a problem, and chose just to answer "DUHHHHHH".
Human brain can analyze better than just come up with a "DUHHHH".