Okay, but if you read written work by an expert (e.g. a book published by a reputable academic press or a journal article in a peer-reviewed journal), you get a

Okay, but if you read written work by an expert (e.g. a book published by a reputable academic press or a journal article in a peer-reviewed journal), you get a result whose details were all checked out, and can be relied on to some extent. By looking up in the citation graph you can track down their sources, cross-check claims against other scholars', look up survey sources putting the work in context, think critically about each author's biases, etc., and it's possible to come to some kind of careful analysis of the work's credibility and assess the truth value of claims made. By doing careful search and study it's possible to get to some sense of the scholarly consensus about a topic and some idea of the level of controversy about various details or interpretations. If instead you are reading the expert's blog post or hastily composed email or chatting with them on an airplane you get a different level of polish and care, but again you can use context to evaluate the source and claims made. Often the result is still "oh yeah this seems pretty insightful" but sometimes "wow, this person shouldn't be speculating outside of their area of expertise because they have no clue about this". With LLM output, the appropriate assessment (at least in any that I have tried, which is far from exhaustive) is basically always "this is vaguely topical bullshit; you shouldn't trust this at all".