In non-horror games it's easy to use combat as the defining gameplay mechanic because there's no need to keep mysterious the beasts lurking in the shadows. In horror, the moment when a monster becomes familiar is when the tension and dread begins to deflate. There's no limit to fear of the unknown, but as soon as an enemy is quantifiable the boundaries are drawn. Fear will eventually become supplanted with frustration or, worse, tedium.
via gamasutra.com