somewhere deep on a shroomery binge one night i came across discussions about different alkaloids being responsible for the nausea. psilocin and psilocybin themselves aren't the culprit, so it has to be something else that's in the fruit bodies. without knowing what that something else is, though, it's rather difficult to isolate away from it in any kind of extraction.
similar to poppies and mescaline-containing cacti, except in those cases the alkaloids one aims to extract are much easier to isolate without picking up unwanted alkaloids vs mushrooms where we're dealing with very small amounts of the active compounds we desire and an unknown quantity of compounds we don't.
the nausea thing is not unique to psilocybes, though. technically any edible mushroom should be cooked before ingestion. shitakes, portabellos, oysters, lions mane, etc all contain compounds that can cause stomach issues to varying degrees, and those compounds are all volatile at the levels of heat we typically cook at. wouldn't surprise me if psilocybes are the same.
to add to this theory, wood loving actives are known to cause wood lover's paralysis, and some unofficial crowdsource research has indicated taking benadryl (diphenhydramine) can counteract or entirely reverse the paralysis. going on that path indicates that it's likely a histamine that is causing the nausea, and in wood lover's case, paralysis, and a simple antihistamine is all that's needed to alleviate those symptoms.