I'm going to agree with the OP on this one, because, regardless of the particular style of horror you're trying to run (gothic horror, survival horror, etc.), all horror basically boils down to the same basic premise -- you are not important. One great example I saw earlier in the thread points this out
Like, I once managed to evoke tremendous horror just by describing a giant's castle in a lot of detail, and emphasizing how much bigger everything was than the PCs. They weren't confused, I wasn't delaying reveals, or even making them wonder what was around the corner - they knew what was going on and were relatively safe in their invisible exploring, but it was sufficiently upsetting that they became horrified anyway, and had things gone pear-shaped (which was unlikely but possible), they knew they were totally stuffed, which helped reinforce the horror.
By wandering through a giant's castle, seeing how everything is on a scale much larger than the PCs can make use of -- it's clear that the space they're in isn't meant for them, and that realization produces horror.
D&D, particularly 5E but to some degree in nearly every edition, tells players that their PCs are heroes, special, important people. Their actions have consequences, can change the world and decide the fate of nations. The rules tell the DM to consistently have the same actions produce the same results, so the characters can not only predict how their actions will have an effect in the world, but so that they can ultimately achieve mastery over that world by understanding the rules and how they allow the PCs to achieve their goals. Horror, whatever the flavor, dies under these presumptions. You may be able to provoke horror in specific moments, when describing a giant's castle or the underground lair of a pod of illithids, but it won't stick -- as soon as the players figure out the boss giant is AC 19 and has 230 HP, they know they've got a shot of defeating it, and a pretty good idea of how good a shot they have.
If there is one aspect of 5E D&D that could be used to produce a horror game, it's this:
(in the midst of a Road Runner cartoon, the camera cuts to two kids watching the cartoon)
Kid 1: You know, I feel sorry for the coyote. I wish he would catch the road runner.
Kid 2: Yeah, but if he did, there wouldn't be any more cartoon.
Kid 1: Oh, right.
Regardless of how many cultists or monstrosities the characters defeat, no matter how many times they save the world, there will always be more enemies, more evil plans, more chances for the world to be corrupted or destroyed. And that, ultimately, ties back to one of the classic themes of cosmic horror: no matter how many times the heroes seem to succeed, no matter how often they appear to defeat their enemies, those enemies don't go away; they simply retreat into the shadows to emerge once they've gathered their strength again. The heroes are ultimately powerless to defeat evil, because if they did so, there wouldn't be a game anymore.
That is the kernel of horror lying at the heart of all adventure games. And it's one the players and DM can never really accept, because if they did, it would render their adventures pointless and their victories hollow. But all the same, it's there, waiting to be discovered.
--
Pauper