Lovecraft said the following about his storytelling style in a letter to fellow writer Clark Ashton Smith:
My own rule is that no weird story can truly produce terror unless it is devised with all the care & verisimilitude of an actual hoax. The author must forget all about "short story technique", & build up a stark, simple account, full of homely corroborative details, just as if he were actually trying to "put across" a deception in real life—a deception clever enough to make adults believe it. My own attitude in writing is always that of the hoax-weaver. One part of my mind tries to concoct something realistic & coherent enough to fool the rest of my mind & make me swallow the marvel as the late Camille Flammarion used to swallow the ghost & revenant yarns unloaded on him by fakers & neurotics. For the time being I try to forget formal literature, & simply devise a lie as carefully as a crooked witness prepares a line of testimony with cross-examining lawyers in his mind. I take the place of the lawyers now & then—finding false spots in the original testimony, & thereupon rearranging details & motivations with a greater care for probability. Not that I succeed especially well, but that I think I have the basic method calculated to give maximum results if expertly used. This ideal became a conscious one with me about the "Cthulhu" period, & is perhaps best exemplified in The Colour Out of Space.
(From a letter dated Oct. 17 1930, reprinted in Selected Letters, Volume III)
I think this could help explain the phenomenon you mentioned--to work as a "hoax" all his stories have to involve only a fairly small number of eyewitnesses and a return to the status quo at the end, rather than, say, a monster going on a rampage that devastates cities, or anything else that would lead to major newsworthy consequences. So, the stories generally have to end with the monsters going back to sleep/hiding or leaving the Earth/our dimension, having only affected the lives of a few people.
That said, the human actions you mention aren't always the main reason the monster in the story failed to go on a wider rampage (spoilers to follow):
- In "The Call of Cthulhu", being split apart by the ship perhaps caused Cthulhu to temporarily retreat to the island, but it's suggested that the reason he didn't return was because the island sunk again while he was there:
Cthulhu still lives, too, I suppose, again in that chasm of stone which has shielded him since the sun was young. His accursed city is sunken once more, for the Vigilant sailed over the spot after the April storm; but his ministers on earth still bellow and prance and slay around idol-capped monoliths in lonely places. He must have been trapped by the sinking whilst within his black abyss, or else the world would by now be screaming with fright and frenzy.
In "The Whisperer in Darkness", the Mi-Go never really seemed to have plans to take our world for themselves, they are just interested in getting some humans to volunteer to be their traveling companions in exploring the universe, which unfortunately requires the humans to have their brains extracted and put in metal cylinders to survive the rigors of space travel, which can be hooked up to sensory devices and speech synthesizers. Since the old farmer Akeley was their target to convince to join them, it makes sense that they wouldn't have killed him, and the ending reveals that they did manage to get him to give up his body.
In "The Dunwich Horror" your description is basically accurate, but the main monster, horrible as it is, seems to be basically a child, and is banished by an incantation from the same powerful book (the Necronomicon) that brought it into this world, shouting for its otherworldly "father" Yog-Sothoth as it goes:
"Eh-ya-ya-ya-yahaah—e'yayayayaaaa ... ngh'aaaaa... ngh'aaaa ... h'yuh... h'yuh... HELP! HELP!... ff—ff—ff—FATHER! FATHER! YOG-SOTHOTH!..."
- In "The Haunter of the Dark", the entity known as the Haunter of the Dark was said to be "awaked by gazing into the Shining Trapezohedron" which Blake found at the church, and it seemed to be primarily interested in pursuing the person who had looked into the Trapezohedron, in this case Blake, who had an "unholy rapport" with the entity. Then during a power outage the entity seemed to break out of the church and fly to Blake's home, and the next day Blake was found dead with a look of horror on his face, suggesting the entity had succeeded in its pursuit. After that the Shining Trapezohedron was thrown into the waters of Narragansett Bay by Dr. Dexter, so no one else would look into it and inadvertently summon the entity to pursue them as Blake had.
