Why are there pianos and mailboxes in the room in horror movies?

Not all horror movies have pianos.

Thrillers not only have pianos, but also use ordinary musical instruments. As long as the method and rhythm of playing are changed, they can play a magnificent or ghostly soundtrack. For example, the low and sad timbre of cello, the roaring anger of metal instruments, the sharp and agile sound of electronic synthetic instruments and the crisp and loud sound of organ can all create a terrible atmosphere, which makes people feel depressed and afraid.