I believe in fairies! I do! I do!
...Except I don't, and neither do you.
"Why don't you believe in fairies?" I ask.
"What an absurd question," you retort, "it's because fairies don't exist." However, the many times when you thought something so-obviously-didn't-exist and were proven absolutely wrong suddenly pop into your mind. "Okay, let's do this exercise then. Why do I believe fairies do not exist?" For the sake of argument, let's equate fairies to diminutive humanoids with insect-like wings that can fly, and leave magic and wonder and weird golden powder out of the equation.
There are certain beliefs you hold about the real world that you deem incompatible with the existence of fairies. For instance,
The widespread knowledge of the existence of another species different from humans but with equal cognitive capabilities would mean that everybody would know about them and whole institutions would be built around them (maybe even countries).
If fairies existed but there was not widespread knowledge about them, it wouldn't take long for them to gain worldwide recognition after their discovery. The only way that would not happen is for the existence of fairies to be intentionally shrouded in secrecy, or for them to not have been discovered at all.
Even if the existence of fairies could be kept secret, it would be extremely rare for nature to spontaneously generate humans AND fairies. Given the extreme likeness and cognitive abilities of fairies and humans, humans would likely be closer to fairies than to any other primate. The divergence would have likely happened very recently, after nature invented big smart human brain, so you cannot help but wonder what selective pressures and coincident random mutations could have led to such a drastic reduction in size and change in function (wings) in such a short time.
"Whew," you think, "I can say for sure now that fairies don't exist." Your belief in the nonexistence of fairies actually paid rent. "But, did I really have to go through all of this just to prove to myself that fairies didn't exist?" Never in your life did you need to go through this tedious exercise to actually believe fairies didn't exist. This is the first time you had even thought of these arguments. Why exactly, then, did you believe that fairies do not exist?
I would place my bet on two reasons:
Everyone else seems to think that fairies don't exist, with extremely high confidence. You're pretty sure everyone around you would be willing to bet $100,000 that fairies don't exist, so you would be willing to bet as well. It would be extremely surprising for you if someone with a sound mind would be unironically willing to bet a large sum of money in favor of fairies existing.
You do not believe in fantasy mumbo-jumbo. Fairies, along with wizards, dragons, mermaids, gnomes, elves, half-elves, half-elf-half-gnomes, quarter-elf-quarter-gnome-quarter-elephant-you-get-the-idea's are all children of fantasy stories, and it seems to you many times more likely that they were invented by some buddy with a florid imagination than Mother Nature herself.
This last point strikes you as especially convenient. If someone comes in and asks you, "Hey, I know you don't believe in fairies, but what about trolls?", you would have to do that exercise all over again but now specifically for trolls, and the arguments for fairies do not necessarily apply for trolls (trolls might be non-primates, for example). That is, if you couldn't invoke the "it's all fantasy" card and be done with all those possibilities in one swoop.
I'm a medical graduate, and recently many people have come to me to ask if "taking X is good for my health" or if "I should eat Y in the mornings." Maybe the ideal response is "I don't know, but let me get back to you after going through the available evidence." Unfortunately, scholarship is a time and effort-wise costly endeavor, and there are other topics I would rather prioritize being scholarly on. So I start crafting my fantasy card with:
If X had been proven good for your health with little room for doubt, it would have reached the ears of me or my peers, because I can't imagine something being definitely good for everyone and not being adopted into standard health practices. The same applies if huge effect sizes had been seen for X, even if they did not apply to everyone.
X strikes me as more likely invented by a tradition not sufficiently grounded on empirical observation.
And then I slam that card on the table and proclaim "I don't think it's going to do any good," whether you ask me about vitamin X, fairies, or trolls. But then I come across topics like these getting good evidence reviews and signal-boosts, and I get confused because "what supplements should I take" was exactly the kind of question that would make my fantasy detection alarm go "ding".
I can always increase the precision of my fantasy detection (with the aforementioned scholarly costs), but I'm left wondering if having that fantasy detection boundary is even justified enough. I think it is. Not everything is worth investigating without enough of an intuition that it can be useful or promising. But I suspect this fantasy card is also why physicians can often come off as fantasy-forbidding grumps who say "no" to every sufficiently nonstandard treatment suggestion.