Single-Player doesn’t run the whole world at once. Most PCs couldn’t handle that. So the world runs only your immediate surroundings while the rest of the world hibernates until you return to the vicinity. Then the game loads that area from zero, meaning that stuff resets.
So if you want to clear a large camp, you can’t leave the area if you don’t want it to reset.
As DaVice said, you can run a Dedicated server just for yourself.