There are a number of reasons why LLMs are "unreliable" (prone to hallucination and bullshitting, as well as just being stochastic).
I think the core issue is perhaps that they don't have any personal experience, nor know the trustworthiness or core competencies of their sources. Their training material is just an undifferentiated mass of text - some might be the writings of Richard Feynman, some the writing of someone on 4chan.
Us humans know the value of personal experience and expertise. If we have seen or done something ourselves, applying our own learned skepticism and knowledge, then we treat that as much more trustworthy than something posted on 4chan. If we read something in a book by a well known expert, or hear it from an acquaintance we have learned to trust, then we also treat that as likely to be true, although perhaps still not as much as if we've seen/tried it for ourselves (for which there is little substitute, since only then can we experimentally test any surrounding assumptions for the transmitted knowledge to put it in context).