People used to eat anything and everything out of desperation. They weren’t out to taste anuses specifically. Well, most people weren’t. They get to the last part, are still hungry, grimace and put it in their mouth and there’s an unexpectedly pleasant (given the circumstances) surprise.

We also have dogs. After enough time with enough people and enough dogs, eventually someone’s going to notice their dog being especially excited about eating the part that they’d normally give to it to eat, and so sniff, if not actually taste. Same surprised reaction.

There’s also the fact that no person (or dog) must have died from doing this, so other people were less afraid to try it.