>>24846308
Probably because horror fiction mainly consists of supernatural horror, with only a small percentage being non-supernatural horror. It also doesn't help that those kind of stories sooner fall into the genres of thriller or transgressive fiction, thereby not completely being seen as horror.
For example, Stephen King's Misery and Thomas Harris' The Silence of the Lambs can be seen as both horror and thriller, and Ryu Murakami's In the Miso Soup and Cormac McCarthy's Child of God both fall under transgressive fiction, even though I'm fairly sure most horror fans would definitely enjoy reading those novels.
Another big aspect that plays a role here is that a lot of psychological horror novels still use a small bit of supernatural horror in their stories, automatically making it supernatural horror despite the main horror being psychological. Some examples that I can think of right now are Ira Levin's Rosemary's Baby, Roland Topor's The Tenant, Thomas Tryon's The Other, and Nathan Ballingrud's North American Lake Monsters.