Well heres the thing, When the Eternals came out Erich Von Daniken was all the rage. He wasn't taken seriously by scientists but his book, Chariots of the Gods, was selling like hot cakes.
What is Chariots of the Gods? Well its the astronaught theory nonsense that the Ancient Aliens TV Show is based on. Apparently Kirby was a fan.
now before anyone says this is hogwash, what does it say on the cover of Eternals #2? "More Fantastic Than Chariots of the Gods!".
Jack Kirby got his ideas from ancient astronaut theory.