My daughter often talks about alicorns and sometimes pretends to be an alicorn. That's a cross between a pegasus and a unicorn, i.e. a unicorn with wings. So why can't she just be a unicorn with wings? Because unicorns don't have wings, of course. Not real unicorns (according to My Little Pony, that is, and perhaps other sources as well).
Someone who is asked how to kill a vampire will probably say a stake through the heart, and perhaps will mention other methods; but will most likely not qualify the statement with anything like "typically" or "according to the majority of popular sources." When it comes to some of the finer points of vampire mythology, like whether they can survive in sunlight, there is significant disagreement among popular sources (The Hunger: yes; Buffy the Vampire Slayer: no; Twilight: yes, but they prefer to avoid direct sunlight because it makes them glitter conspicuously). It wouldn't be unheard of for vampire geeks to argue about such things without acknowledging that they are really arguing about which mythology is the dominant one, or which mythology each person prefers. Like when I was talking to someone who claimed that "Elvish" and not "Elfish" is the correct way to refer to the language of elves. ("Says who?" I said. "You realize elves aren't real, right?")
Consider, by way of contrast, Sherlock Holmes. He is no more real than unicorns, vampires, or elves, but if you want to know his address, there is a definitive answer. Or at least there is an answer that virtually everyone can agree on, because virtually everyone would agree that Arthur Conan Doyle is the definitive source. This even though there may very well be fan fiction out there in which Holmes lives at 221A Baker Street, or 1600 Pennsylvania Avenue, or wherever. In the same vein, one can confidently claim that J.R.R. Tolkien is the definitive source for information about hobbits. He did invent them, after all. There is much more room for argument about elves (and orcs, and all kinds of other stuff), not least because there won't be universal agreement about what "definitive" even means. (Tolkien also capitalizes the names of these races, but I'm not on board with that. Languages, on the other hand, even made-up ones, should be capitalized, imo.)
Here comes the tortured analogy. Nowadays, knowledge is viewed more like the mythology of vampires or elves than that of Holmes or hobbits. I think that's fair to say. There is good reason to view results of scientific research as authoritative (which is not to say infallible), whereas arguing over whether one non-scientific source is
more authoritative than another tends to boil down to declaring
allegiance to something--an ideological group, a way of thinking, a set of values, etc. Which is something like claiming that one depiction of vampires (or elves or unicorns) is "right" and another "wrong." Far from a perfect analogy, but what the hell.