"Backrooms" don't just come from videogames. They are meant to represent liminal spaces like "endless" cubical farms and conference rooms and the back offices of movie studios or any other modern business. (Even the idea that on the backside of the cool theme park structure that seems so otherworldly is just a couple of boring janitor's closets and hallways for staff/crew to navigate between shifts.) The videogames building "unused" rooms like this were in part trying for verisimilitude to these sort of "just around the corner" spaces that exist in so many buildings. Often as a joke. It was a part of the humor of Duke Nukem. It was a key part to the humor of Portal. It was the entire basis of The Stanley Parable.

I think we can argue that real world places that inspired our fantasy Dungeons were similar liminal spaces: the creepy basement hallways that connected staff/crew (servants) access to other parts of the building(s) above. The multi-use spaces below that are most remembered in pop culture for such uses as torture and imprisonment, but were also often staging grounds for much more boring household logistics tasks (storage), and even equivalents to conference rooms, janitor closets, and "offices".

This feels like a silly over emphasis on a naming that ignores how alike it is to so many things that came before. Don't even have to go too far back to get stories of people finding themselves in a fantasy world through a wardrobe.

How many stories were about hidden worlds below our own? Isn't even that much different from "turtles all the way." Heck, even the Minecraft movie played with a literal mine going into a magical world.