The things you said about horror movies compared to books might touch on what the issue is for me personally. I think part of the reason I don't really see King's books as "horror" is that I associate that term and that style of storytelling with horror movies. I associate it with stories that have gore and gotcha scares and techniques to manipulate the emotions to freak the viewer out, as you mentioned, not to mention the whole torture sub-genre that's cropped up. Those are all the things I think of when I think of the horror genre, because of my negative experience with horror movies.

(Though it should be noted that when I watched The Ring, which is a fairly well-regarded horror movie, it didn't really scare me or do anything for me, and I didn't much care for it as a source of entertainment, so again it might be fair to say that I don't react to the genre in the same ways most people do.)

Since I associate the term horror, at least as it relates to storytelling, with horror movies, that could explain why King doesn't quite feel like horror to me. As you noted, he's a little different in how he goes about achieving his goal of scaring people.

Given that I agree that King can write tense and sometimes disturbing scenes that put supernatural and dangerous twists into regular American life, it might come mostly down to definitions. I don't associate King with horror because I don't associate him with horror movies. But I also haven't read much in the way of horror novels outside of a few Dean Koontz books of my father's when I was a young teenager. If the horror genre within books is different than the horror genre within movies, then that alone might explain everything I feel on the matter.