It doesn‘t know that it doesn‘t know because it doesn‘t actually know anything. Most models are trained on posts from the internet like this one where people rarely ever just chime in to admit they don‘t have an answer anyway. If you don‘t know something you either silently search the web for an answer or ask.

So since users are the ones asking ChatGPT, the LLM mimics the role of a person that knows the answer. It only makes sense AI is a „confidently wrong“ powerhouse.