For adults, though, I see "Why?" as the lazy question. It throws all the effort of actually formulating an answerable question on whoever tries to answer it.

I think this touches on something huge. Asking why, and seeking to answer it yourself, is not childish or annoying or looked down upon - it's science! Asking why, and expecting someone else to figure out the particular question and find an answer for you is absolutely childish. It may in fact be appropriate for a child, but it does hit a point where the work being demanded is far more than is appropriate to the interaction, and difficulty in dealing with this leads to frustration; it is not easy to explain to a child why asking "why?" and expecting the adult to provide a response in the context of a short interaction was reasonable the past 4 times and now is unreasonable.

Note that while I would say, "answer this for me" is generally childish, "come answer this with me" is more often not - I don't mean to exclude involvement by others. In all cases, though, it depends somewhat on circumstance.