Back when men believed the world was flat, it was said that whenever an explorer sailed off the edge of the world, a Drifblim was born. Today, it is known the world is a sphere, but there are still men who push its boundaries, and a pokemon once thought to be the ghosts of lost sailors are now the ghosts of those who die in aviation or astronomy accidents. In any age, Drifblim are the spirits of people who lose their lives pushing the boundaries of the known world.
When these restless, inquisitive spirits become Drifblim, they continue to explore by traveling around the world through uncharted territories, often going high enough to meet Rayquaza in the ozone layer. But Drifblim do not maintain their memories, and so the towering cities of men are to them as mysterious as anything in the pokemon world. Together with those who perished with them in life, they glide towards Goldenrod or Castelia, an aerial flotilla of the dead.
Yet humans have created weapons of aerial war based on pokemon such as Latias and Drifblim and used them to bomb each others' cities, and many explorers were themselves cruel tyrants seeking to subjugate unknown lands. It is not unheard of for a curious group of Drifblim to be mistaken for an invasion fleet, or when the city they find has few weapons, to become one for real.
It is, however, far more common for the Drifblim to simply float away. Many manage to escape Earth's atmosphere outright and take up residence on Earth's Moon, where their floating does not mark them as unique, which men have always sought to explore, and which many exotic pokemon call home.