As an author, I can say from experience that horror books are NOT easy to write. On a screen, you can utilize images, music, camera angle, and SO many other things to scare a viewer. On a page, all you have are your words. So, to be able to truly scare someone with only your words is a real accomplishment. For this reason, it is a 
considerably smaller genre than some others, but that doesn’t make it of any less value.
There are a huge multitude of amazing and truly scary horror books out there, many which are not getting the attention they deserve, even within the horror-loving community.