I feel like in real life, we're constantly exposed to "I don't know" as a valid answer, but obviously we don't write down all the I-don't-knows in expert literature so the training corpus is wildly skewed towards confident answers because "we studied this for a month and have no idea, it's confusing" doesn't get published.

That's a great point. A training corpus based on written text will be inherently biased toward confident and right. Then the RLHF exacerbates the problem because people respond more positively to confident and too often assume correct when they read a confident response.