I know this is a joke, but I think it's important to recognize that it's because the ChatGPT language model does not have the ability to introspect and decide how accurate its knowledge is in a given domain. No amount of training on new input data can ensure it provides accurate responses.
No it doesn't, humans can recognise when they don't know something, current language models usually can't (yet)
Their training objective, which is to predict the next piece of text in their training data, does not incentivise them to respond that they don't know something, as there no relation in the training data between the AI not knowing something and the correct next text being "I don't know" or similar
