>>725083303 (OP)
Like the other anon said, the point is being unable to accept or comprehend your own insignificance.
When Lovecraft and co. were writing, they were writing in an artistic and philosophical period we call Modernism.
This started up somewhere in the mid to latter half of the 19th century and continued to around the middle of the 20th century.
This was a period where mankind was, simply put, extremely self-confident.
People genuinely started to feel that they had the universe figured out, that they had society down to a science, that they had humanity down to a science that could be comprehended totally. All of the universe's mysteries were within reach to be grasped, studied, documented, and understood and then communicated to others. One just had to be confident and serious and put in the work researching it and theorizing.
Cosmic horror was intended to directly counter that.
Timelines for events stretched not just from centuries but to hundreds of millions of years. The scale of conflicts didn't just address individuals but entire worlds. The laws of physics were treated as farces. Humanity's ability to understand things was treated as just entirely insufficient to grasp even the most basic of concepts of what the universe was really like. Man's ability to understand himself was subverted.
Lovecraft was afraid of most things, but the ocean in particular was one thing alien to him. As well, even at the time there was an appreciation that the deep sea and outer space shared characteristics.
So his fantasies about creatures and beings from space or outside of time, naturally came to reflect what he saw as most alien and distant from mankind, deep sea organisms that were basically already space aliens.
His fellow writers went different directions aesthetically at different times, but he was widely respected and emulated, so his aesthetic became the standard.