Because they don't _understand_ things. If I teach an LLM that 3+5 is 8, it doesn't "get" that 4+5 is 9 (leave aside the details here, as I'm explaining for effect). It needs to be taught that as well, and so on. We understand exactly everything that goes into how LLMs generate answers.

The line of consciousness, as we understand it, is understanding. And as far as what actually constitutes consciousness, we're not even close to understanding. That doesn't mean that LLMs are conscious. It just means we're so far from the real answers to what makes us, it's inconceivable to think we could replicate it.