Maybe LLM's know so much that it makes it difficult to feel the absence. When someone asks me about the history of the Ethiopian region, I can at most recall very few pieces of information, and critically, there is an absence of feelings of familiarity. In memory research, familiarity signal is can prompt continued retrieval attempts, but importantly absence of that signal can suggest that further retrieval attempts would be fruitless, and that you know that you don't know.
Maybe knowing so much means that there is near saturation for stop tokens...or that llms need to produce a familiarity like scoring of the key response of interest.