Did you write a vampire novel, maybe one with zombies in it, or powerful mages set in the modern world? Do these characters tell others that it was aliens that built the pyramids at Giza, Chichen Itza, the leaning tower of Pizza, and McDonalds? Then you have a bad case of One Myth to Explain Them All!
This world building trope is common in modern day urban fantasy and sci-fi settings, but not unheard of in fantasy as well. It is much easier for an author to create a world where everyone (or everyone of importance) is a descendant from Atlantis than to try and wrestle with a dozen or more creation myths, mythologies, religions, and other origin stories. But this trope is not without it’s problems. Take the Ancient Astronauts concept. To be blunt, it is racist/bigoted in the extreme. You mean to tell me that the ancient Egyptians, the Babylonians or the Maya could not build their civilizations without the help of extraterrestrials but western Europeans did it all on their own?
So while tempting, this is yet another trope that must be thread with care. A cool idea that can easily lead to unfortunate implications.