Because it’s not designed to be a knowledge base, it’s designed to imitate human communication. It’s the same reason why ChatGPT can’t do maths - it doesn’t “know” anything, it just predicts the most likely word/bit-of-a-word to come next. ChatGPT being as good as it is at, say, writing code given a natural language prompt is sort of just a happy accident, but people now expect that to be it’s primary function.