I mean you kinda just answered your own question. There's no agreed upon Creepypasta universe, the closest we got is the "Slender Mansion" thing, and I think even that is questionable as to how preferred a reality it is.
Creepypasta already is a subgenre - the licenses for many of the most famous stories belong to so many writers already, that there most likely will never be a "shared Creepypasta" universe where Everyone's Here! :tm:
Not unless someone deliberately and carefully reconstructed/recontexualized said famous creepypastas, but in a recognizable and new way. And even then, it'd just be a Ship of Theseus issue, with "is this still a Creepypasta?"
