Why are LLM mistakes said to be hallucinations and not simply mistakes?
A hallucination is a specific kind of mistake. There are also other kinds of mistakes LLMs can make; for example, they can draw incorrect conclusions from accurate information, or they can fail to find information that they "should" know.
How about citing legal cases that don't exist? I would say this is maybe closer to hallucination if an LLM is doing it. Maybe less so if a human is and is not experience some mental instability. What creates that distinction?