Couple this with one of my favorite conceits in fantasy tabletop gaming - that all worlds are connected and reachable from our own Earth - and a lot of how and why questions immediately present themselves. Why are humans everywhere? How do they even share some aspects of Earth culture, but clearly not all? What about all these near-human species that also tend to show up a lot, sometimes with slight differences from the equivalent species on another world? Etc.
The way I currently see it, there are a few broad categories of explanations here.
First of all, these fantasy worlds might have come about in a way that was simply influenced by generic Western fantasy. I think that Codehaven (C0d3H4v3n), from my hopefully-not-abandoned PermaDeath.net effort, is an example that leans this way a bit. Somehow, through existence in a computer, or in a pocket universe created by a human, or...uh, something else, probably...the fantasy world has taken on clichés associated with fictional fantasy settings in our world.
Second - and this one can be connected to that first explanation - the fantasy world(s) might be an actual creation of the human mind in some way. Here, I'm thinking of some stories that feature characters who find themselves in a realm they previously believed to be fictional. The old D&D cartoon might fit this...or maybe something like the Japanese predecessor to Super Mario Bros. 2, Yume Kōjō: Doki Doki Panic, in which a family gets pulled into the fantastic environment of a book.
I guess the thing that differentiates this from the first category is that something like a fantasy world in a computer probably has an origin story that involves specific programming, and some rogue code, and maybe a magical event to fully set things in motion...while this second idea is more along the lines of consciousness and imagination creating reality by their own power. It's really all connected, though. Or, at least, it can be. A high fantasy writer who wakes up to find themselves in a world of their own creation kind of fits both models.
Finally - and this is the one where I get to be extra sciencey - maybe the people and societies of all of these worlds actually branched off from the same original lineage.
I really like this one. It's possible that some of the roots of my obsession(?) with it can be traced to the notion in DC Comics that ancient Atlanteans are connected not only to the Warlord's home of Skartaris, but possibly also to current (or near-current) populations of Kryptonians, Rannians, and Thanagarians. I even worked some details of this idea into the very background of the Paleolithic adventure I ran a couple of years ago (in the overarching potential headcanon, at least). And really...connecting Atlantis-type mythology, our interactions with other ancient human species, and the potential for plane-hopping adventures...well, that's all pretty much in my wheelhouse!
I call it the Fantasy Diaspora - or Fantaspora - Hypothesis...a proposal that the many interconnected fantasy worlds of our fiction and gaming are populated by descendants of ancient inhabitants from our very own Earth. It sounds awfully pretentious, and it's probably been done before, and I love it.
As always, hopefully more to come...