The main explanation Clark offers for things that seem to appear in the world of the Backrooms is that they are like memories, imperfect, constructed out of almost photocopied instructions. You can get close to something, but never quite all the way there. The dog metaphor is used a couple times, and does a good job of cementing this as the thesis behind the structure and substance of what appears in the Backrooms themselves.
To continue to back this up visually, we have the descending shot from Mary's childhood home as the structure of the room seems to warp and degrade, almost as if you were to show the original image to someone briefly, have them describe the room to someone else, have them recreate it, and so on down the line.
Additionally, you have specific memory markers from Clark that seem to have been introduced into the world, but perhaps not originally when he appeared there. You don't really get much specific to Clark's memories (beyond generic furniture) until after he is knocked unconscious by an unseen force and then found several months (?) later by Mary.
Likewise, Mary's memories don't seem to manifest in the world either until after she herself is captured by Async (the degraded storefronts, her own doppel, etc)
My theory here is that Async's MRI machines have something to do with the "why" of how things came into existence in the world of the Backrooms. Within the MRI is an echo, a photocopy almost of the brain's makeup and pathways at the time it is taken. No real soul behind it, just the pure data. There is a potential that this data has effectively been accidentally copied into existence. Imperfect, but yet eerily familiar. The theory would be Clark was captured by Async, scanned, and let back loose in the Backrooms, as it is only after his capture we see Captain Clark, his store replica, etc.
This would also explain why so many shared collective images seem to be there, as it is an amalgamation of many individual memories all manifesting together. Obviously there is no real specific explanation, and I am just ultimately trying to piece together a "why" when that's kind of the point, not to know, but still
I haven't done much research into the Backrooms lore any deeper than the first couple YouTube videos and the original copypasta, so perhaps this theory is nothing new, but still fun to play around with!