The fear in Lovecraft is a fear of the unknown. Of a universe that is so vast that humans can't even comprehend a fraction of it. We are so small and weak, whose to say there aren't things out there that are to us as we are to ants. Massive entities of such incredible age and harboring such unknowable malice for lesser existence that their mere EXISTENCE threatens our species as a whole. That if one of them were to turn an errant eye, on accident, and spy us across the vastness of the cosmos then our fates would be sealed before we could even cry for help. And no power, religion or science, can hope to stay this inevitability. Our only respite is that these beings either haven't noticed us yet, or are having too much fun molding and shaping our fragile lives.
It's the fear of the deep ocean, of great terrors so close and yet hidden from our plain sight. It's a fear of your your fellow man, of the hidden urges and desires held in their heart and your own. It's the fear of knowledge, of the insatiable thirst for understanding that leads to these truths, and the inevitable refuge in insanity that must follow.
Yeah, if you are just focusing on his monsters you have no imagination.