Do you like horror games? How many did you play that really scared you? Not jump scared, just good old fear. Not many, I assume.
So, why is that? What causes fear? As far as I know, fear comes from the idea of something bad happening to us. It may be obvious threat, like a maniac running to you with a knife, or just an unknown noise in the dark. It does not even have to be us who is in danger, just someone we care about. That is what movies and video games are counting on. A good one has likable characters and is able to maintain the illusion of danger.
For video games it is often said that when you can fight the monsters, it is not scary anymore. While there is some truth to that, the scariness is always temporary. The more you get to know the monster, the less you will be scared by it. Games are in a disadvantage here, compared to movies, because while it takes few rewatches for a movie to lose its magic, a game can lose it in the first level. Even when you can not defend yourself in the game, getting killed enough times will replace fear with frustration. And the monster has to do something. If it just stares at you, you will not be scared of it for long.
Fortunately, it does not matter that a horror game is not scary. The horror themes themselves are more than enough, because in the end it is all about having fun. But, if you still want to make your game scary, you will have to find a good balance in how often does the player come into contact with your monster. Good luck.