Why do children ask why constantly?
Because it works. Around age three to five, children discover that a single word reliably produces information, and they are in the most information-hungry period of their lives.
Research into children's questioning shows this is not idle noise or purely attention-seeking. Recordings of preschoolers' conversations find that when children ask why, they are more satisfied by explanations than by non-explanatory answers, and they are more likely to ask a follow-up when the answer does not actually explain anything. That pattern only makes sense if they are genuinely after causal understanding rather than engagement.
What is developing underneath:
Causal reasoning. The child is building a model of how the world hangs together. Every "why" is a probe for the mechanism connecting two things they have noticed.
The realisation that adults are a resource. Learning that other people hold knowledge you can extract is a significant social-cognitive step.
Language for abstraction. "Why" questions require handling causes and consequences, which are far harder linguistically than naming things.
Why the chains go on forever: children do not yet know which questions have deep answers and which bottom out. Following a chain to "because that is how it is" is how they learn where the edges are — and adult explanations often contain unfamiliar words that each generate a new question.
What helps: answer honestly and simply, at the level they can use. Say "I don't know — shall we find out?" when you don't; modelling that is more valuable than inventing an answer. Turn some back — "what do you think?" — which is not a dodge but a way to make them reason.
When it is genuinely connection-seeking, which sometimes it is, the underlying request is for your attention rather than the fact.