I wonder if there's a contractual issue that has prevented this from happening. By all accounts, much like TRON, it gets incredibly high GSATs and while I know some of you don't like it (*cough* @JungleSkip @Joe *cough*), the vast majority of guests do so i'm surprised it hasn't been cloned yet. I do wonder, especially since the contract was drawn up all the way back in 2011, before Disney even bought Star Wars, let alone Fox, if the contract states that any future expansion has to be a land.FOP, by far, is the most significant part of Pandora. Looking at budget, and available space, it makes good business sense to just built FOP and do a small, not very costly, land facade around it.
And to answer questions about the contract since Disney owns Fox now, no, Disney doesn't own that contract. Yes, Disney now owns the 20th Century licensing of it, but 20th Century simply gives James cameron money to make these films and then they distribute them. 20th Century Studios does not own Avatar. James Cameron owns it. 20th Century only owns theatrical distribution rights. Technically, they still have to partner with and pay James Cameron's production company Lightstorm to put the films on Disney+, those are not automatic (although i'm sure a deal is in place). Theoretically, Cameron could take streaming rights and bring Avatar to any streaming service he wants, but that's not good for him in terms of his business relationship. He still needs 20th Century to give him buckets full of money for Avatar 4 and 5.