I find that there are two main things that makes a game really scare:
1) Difficulty- Games where you will die a lot and every second is a desperate fight for survival will make a game much scarier, it immerses you more in the game and makes the situation more serious.
2) Amount of control you have- A main reason movies can be scarier than games is that in movies you are forced to sit there and watch everything, but in a game you can just go around blowing the shit out of everything. If you make the enemy difficult to kill, sneaky, and make the player more vulnerable, it becomes more of an uncontrollable, desperate situation. Then you're scared.
Also, a lot of things screaming and appearing out of nowhere.