What is the age where kids are always asking, "Why?" They reach a point where they discover this word, and the magical power it has over adults, and it is so wonderful to have this power they just go around asking why why why why why? No matter what you say, "Why?" No matter how good the answer, "Why?"
Then at some point they grow up and see the senseless futility of promiscuously asking, "Why?" Some of them it is because they are smart enough to figure out that if they don't specify exactly what it is they want to know the adults can give them any damn story. And your power over them is wasted once you realize it really isn't power at all, they are just playing with you.
Some eventually learn to be careful and respectful and only ask "why" about things where it will actually accomplish something useful. Unless maybe they do just want the adults to play the game.
This I am willing to venture was a lot more explaining than you were expecting. Which only makes me wonder, "Why?"