DS11 never did the "why" questions. In fact, he had problems with questions in general -- understanding the difference between the types. I'm sure it's his Asperger's. I would ask, "how do you..." and he would answer, "because...". It was extremely frustrating, trying to explain why "because" was not an answer to "how".

DD7 didn't start at two, but somewhere along the line (5?) became attached to "why" questions and every other kind, and she still is.