In a way though, they would be "under one roof" in the metaphorical sense. They are all cooperating in order to form the Bizarre Bazaar. There must be some sort of agreement made, in order to prevent hassles and stuff between shops. Just like how the various stores in a mall have to sign agreements, you probably need one for the Bazaar. Also, adding "massive" to the post lets the reader know that the description is talking about a literal roof rather than a metaphorical one, and is meant to show that the bazaar is in doors, in one large building.