I think the backrooms doesn’t represent a fear directly connected to public spaces, but rather the combination of fear and intrigue of the private spaces next to public spaces. It’s that feeling when you’re a kid in the mall and you see a door marked “employees only” or “authorized personnel”; what’s behind that door? Could be something cool, could be something scary, I’d get in big trouble if I went in there but that makes it all the more alluring.
That’s why you see that trope in fantasy and sci-fi all the time: if you knock three times on this side door and then turn the doorknob clockwise during a full moon, instead of opening to a restaurant kitchen it’s a door to a magical realm. Or the bridge and tunnel authority building in Men in Black, unassuming building thousands of people pass a day but if you’ve got the right ID the receptionist will let you into the secret government alien agency.
That’s why backrooms lore revolves around someone going into the infrequently used part of a building and through some rip in the space-time continuum back there where they no clip into the backrooms.