Tbf the very concept of the universe as we understand it is a pretty new thing that we don’t actually know as much about as you may think.

We have known for slightly longer than a century that the Andromeda Galaxy was an entire galaxy, IE, we have only known for a hundred years now that there are any galaxies besides our own.

So that’s basically why parallel universes is such a “new” literary concept, even if ideas of there being other planes of existence predate that concept.