I can't speak for everyone else making horror games, but just from my own experience, one of my first little games/apps was just an experiment in procedurally generating mazes, and once that was working I thought hey, I could put in a first person controller that just walks, keep it dark except for a flashlight, leave it quiet except for footstep sounds and occasional suspense music (stock suspense sound I later heard used a lot in movies) while you try to find your way out. Not quite horror and not quite a full game, but still it was kind of popular and by going that route it simplified development and asset requirements.