I didn't say, "unreasonable." I'll say it's surprising. I'll also agree with the idea that, as
@Joe said, "...a lot of people are lacking imagination or trust with entertainment." And I'll say that for the exact same point you provide.
The "context" of [blank]+horror initiates somewhere. That's true. And while history and tradition can be a component of that initiation, it is not the ONLY initiation. If it were, then everything in horror would based entirely on real-life situations. Every horror movie would be the story of Ed Gein and only that. There would be no Victor Frankenstein. Or Invisible Man. Or Alien and Predator. Or Gillman. Or even zombies. Those things exists because someone looked at something WITHOUT context and applied "horror" to it. It wasn't done for them.
If someone has to rely on (as you say), "the familiarity that's already in place," the implication is that that someone cannot create that relationship on their own; ie - they "lack imagination."
And it goes even further than that. Most "horror" (as an additive to any situation) is based on empathy - looking at something and imagining how scary or painful it would be to have that happen to you. The Purge is scary because we imagine walking through
our streets with everyone out to kill us. Hostel is unnerving because we imagine ourselves getting tortured. Freddy Kruger is terrifying because he can invade our own dreams. That's why it works. Even if we apply that to history, Ed Gein repulses because we imagine someone we know doing the same thing. The Donner Party compels because we wonder what would drive US to do that. However, if you don't know who Ed Gein is, if you don't know who the Donner Party is, your failure to empathize and create the horror associated with it is on you; its not on Ed Gein or the Donner Party.
So, if you don't know that Vikings' fought for the glory of their gods of War and Death, if you don't know that the term "berserker" is based on their fighting style, if you don't know they raided as far as Baghdad and Canada - destroying villages and taking slaves as they went, then you wouldn't be able to (again) imagine any of those things happening to YOU.
Roanoke is a fantastic example. On paper, it is literally, it is "pioneer settlement overrun with wendigos." What's a wendigo? It's a spirit that makes people cannibals. And? Do you know what a cannibal is? Why is it based after a city in Virginia? If you don't know what any of those things are, of course you won't be able to apply the "horror" filter. That requires YOU to fill in the blanks and provide your own additional context in order to understand how it works.
EDIT - Skepticism for how something can be presented is completely justified. Skeptism toward how something can be "horror," sorry, says more about an individual.