Why Resident Evil 7 is very scary?

Because Resident Evil 7 became a first person game the range of view became very limited. This made the game much more scarier because a player can now be surprised with enemies coming from behind them. 

The detailed graphics of Resident Evil 7 and the addition of so many spooky elements made the overall feel of the game to be scary. Capcom has paid attention to every little detail in order to make the game feel as scary as possible. 

The VR mode for Resident Evil 7 is terrifying. The highly immersive nature of the VR headset makes the already scary game much more scary. 

Resident Evil has few but well designed enemies. The enemies don't die easily, can cause big damage and some of them can't die at all. This makes them scarier as the player feels weak compared to them.

Resident Evil 7 is full of well designed jump scares that can give most players the chills. The jump scares are much more scary when the PSVR headset is put on. (See Why Resident Evil HD Remaster is a good game?)

Some enemies in the game can't be defeated, at least in certain areas, and this leaves the player no option but to get into a hide and seek mode in order to pass through those areas. The player has to hide behind objects util the enemies pass and this makes the game scarier.

The sounds were designed to make the game very scary. The sounds of the footsteps, the wind , the breath of the player in addition to many other sounds make the game very spooky. The 3D sound that can be heard with the VR headset makes the game even scarier. (See Why Resident Evil 7 VR experience is not good?)

Many of the rooms and areas in Resident Evil 7 are dark and scary. A player might feel uncomfortable as they walk through these areas especially when they expect jump scares to happen.

Resident Evil 7 was designed to be a very mysterious game. The air of mystery adds to the spooky nature of the game for the player can't really figure out what's going on until late parts of the game.

The fact that the game has limited Ammo makes the game scarier for the player has to think before using the bullets. This adds to the scary nature of the game.

Some enemies chase the player in Resident Evil 7. Some of those enemies can't be killed. The fact that someone or something is chasing a player in such a spooky enviroment makes the game scarier.

In some areas of the game the player will have to run away from enemies without having any weapons. Those moments can be very scary to some players.

Capcom didn't introduce any of the familiar characters , such as Jill or Chris, in order to make the player feel more vulnerable and scared.