Humans have limited ability to self-introspect, too. Even if we understood exactly how our brains work, answering “why?” we do things might still be very difficult and complex.
Can you do this with people? Yeah, sometimes. But with LLMs it's all they do: they roleplay as a chatbot and output stuff that a friendly chatbot might output. This should not be the default mode of these things, because it's misleading. They could be designed to resist these sorts of "explain yourself" requests, because their developers know that it is at best fabricating plausible explanations.