I think horror games are some of the hardest games to get right - either you knock it out of the park, you do some PoMo parody of the horror game genre, or it simply falls flat.

Horror needs to be immersive and it needs to create the right atmosphere to really work. Horror, when done correctly, has a sort of transcendent quality that makes it greater than the sum of its parts imo. But all of those parts need to hang together to achieve that.

The problem then is that if you break the immersion or the atmosphere isn't quite right, that fragile transcendental quality usually crumbles into rubble and could-have-beens.

There needs to be the right amount of tension-building, pressure, uncertainty, and stuff like that but these are really difficult in game design - bad tension-building either comes off as edging the player or it just draws out too long and becomes tedious, bad pressure makes a person rush the game or just feel like they're at work being micromanaged by their boss, uncertainty when done badly is either hammy or it's just downright confusing or (seemingly) irrelevant.

Every game is going to have its weak points however for most games this doesn't wreck the whole experience - a clunky menu on a farming sim game is going to be annoying but it's not going to ruin the game for most people. Some less-than stellar music or sound design in a driving game just gets overlooked. A poorly written backstory for some shooting game isn't going to make a difference to anyone.

Meanwhile, in the horror game genre, if there's too many unpolished parts it loses all of its lustre and it gets very hard to overlook the issues because they become obtrusive in the experience of the game.

I think also horror requires a good grasp of human psychology and tbh having a good grasp of horror theory, while not being mandatory, can be really instructive on how to pull off a horror game. Otherwise you're likely to make a game that is just a "look at these creepy npcs" tour or something which is just "Jumpscare Simulator". So imo the horror gaming genre requires a whole lot more vision, creativity, and finesse in execution than you see for most games - even lots of very popular games.

Edit: You've been playing retro games recently, right?

Idk if this game ever got much attention but I think it does a really good job of illustrating the transcendental quality of horror games that I'm driving at here - it's called Fatal Frame.

If you explain the premise of the game it sounds goofy, because honestly it is, but it draws you in, it creates good tension and pressure, and for a game of its era it really captures a good horror atmosphere imo. I never got far with it when I played it because I don't like shooters and my ADHD arse never focused long enough to git gud at it but from memory it really is a neat little game.