Alternatively, the prior on "this is not possible" is very low because RLHF & Friends have targeted metrics that, inadvertently or not, discourage that outcome.
Dataset as well. In a forum if you don't know the answer you simply don't post. Only people who think they know will post an answer. In a dialogue you see a lot more "I don't know" since there they are expected to respond, but there isn't a lot of dialogue data to be found on the internet compared to open forum data.