system
A chat on subreddit /r/NoStupidQuestions
human
A number of reasons.

Firstly, it is genuinely too difficult to explain. For example why when water flows down a gentle slope why does it have stripes. But I can't think of a simple way of explaining it without mathematics. 

Secondly, I can't be arsed to explain.

Thirdly, it might involve having to explain something I don't want to explain. Eg Where do babies come from.

Forthy, it might involve explaining feeling and emotions that the other person will instantly reject as invalid and argue with you about. Eg Why don't you talk about your feelings.
gpt
A fifth reason might be that it's something they've explained numerous times to numerous people and they're sick of repeating it. Suppose that can fall under the second reason, though, but then they all can really.