Sort of an aside: I find it so strange to insist that everything have The One True Story With All Its Details And We Shall Murder Anyone Who Disagrees ("canon")

Sort of an aside: I find it so strange to insist that everything have The One True Story With All Its Details And We Shall Murder Anyone Who Disagrees ("canon"). I'm not an expert in literature by any means--I was bad at, and hated, lit crit--but it seems like this is a relatively recent invention, or at least that its application to anything other than religious texts is a recent development. For starters I don't think this is how stories have been told for most of human history. Before Beowulf was written down, were people interrupting scops to say that the color of the dragon in their version was non-canonical? And even if they were, most stories--and especially ones that we seem to get bent out of shape about when it comes to "canon"--aren't real, so to insist that there is some set of essential, unchangeable facts that they refer to is gibberish. Would very much appreciate insight on this from someone who has studied literature and/or folklore extensively (though I suspect this might not be the right place to find such a person).