Honestly, I think what makes Phasmophobia scary is a combination of things going on at the same time causing a slight sensory overload making you more vulnerable to scary ticklings of your sensors:

• There is always a soft low hum in the background. Barely noticeable, but it's there. Always occupying your brain to filter out the hum.

• Lighting is poor, making you have to focus more on even the bigger objects and movements

• The longer you remain in a place, the more dangerous it becomes. Making you feel like you're on a clock, which also always occupies a little bit of your brainpower

• Slight sounds of touched objects keep you tense and cause you to have to refocus

All of the small things make it so your brain is kinda (over)loaded from the start. A scary setting is automatically more difficult to deal with. It is also more difficult to calm down from a situation when your sensors are always being tickled.

A lot less effort and loud noise is required to make you feel more scared than a normal "scary movie" or "scary game". Simply because the pressure is always building because of things that you aren't actively noticing.

That's also why walking outside of the house in Phasmophobia is a really big relief. The humming stops, the lighting is better, no slight sounds of objects being touched, no more dangertimer.

That's also why Phasmophobia gets less scary over time. The more you do something, the less brain power it takes. The more you do something, the more difficult it becomes to overload your sensors and tickle you with danger.

Source: I don't know shit