Why questions are very good and should be encouraged! But also, it is worth improving the questions, in addition to just answering them. So if a child asks "why is the bus going?", you can ask for a clarification "Do you mean what makes it move? Or do you mean where is it going?"; this models clearer language and better communication skills, it helps the child get an answer to the specific question that they intended, and it prevents why-questions from becoming the default I'm-bored-so-I-will-say-why-until-people-get-sick-of-talking-to-me routine.

Sorry, I know that was slightly off-topic.