I like the title OP! Catchy
As you said, ancient people spread amongst the globe over, with no immediate physical contact/communication, share a lot of similarities, in the
dragon, great snake etc..
Then we know we had dinosaur monsters roaming casually and flying in our space, although a different "time" period.
Focus on, what does every people have in common and share, no matter what region they are inhabiting?
Sub conscious mind. Dreams. Visions. Internal worlds of our possible interdimensional selves.
Everyone could "see" (minds eye, third eye, mystic eye etc) dragons, fairies, elves, trolls, goblins, unicorns, reptilians, greya, demons, angels,
whatever - beings from another realm not seen with two eyes.
How all these different cultures throughout time can have so much simular belief structures, idols, gods, magical
creatures etc.. is explained
I have seen several of those beings I listed through my minds eye, it's a natural function and ability of the mental faculty everyone has.
So did they leave? No, people stopped seeking internally and got disconnected. Which is very evident if you view the human species from