There is a certain cliche of a young child asking "why?", getting an answer, asking "why?" to that, and so on until the adult finally dismisses them out of frustration.

Simple miscommunication. Kids figure out that saying "why?" causes people to tell them interesting, related things. So they keep doing that. The adults who hear this, however, are applying an additional constraint - the things have to be related in a particular way - which the child probably doesn't care much about. Unfortunately, that constraint sometimes leads to dead ends; there are many concepts where all the answers to "why" are either unknown, or uninteresting, or built on prerequisite knowledge not available to small children. The graceless way to handle this is to get frustrated and dismiss them. The right way is to segue the conversation to something that isn't related by an is-cause-of or is-purpose-of relation, but which is interesting and worth talking about.