The land determine the pokemon though. Remember each region has distinctive Pokemon and other pokemon can't be found there. Kanto your starters are Bulbasaur, Charmander, and Squirtle, yet in Hoenn which is my personal favorite region those Pokemon are nowhere to be found. Remember IP integrity is also important and a real thing not just lets put thing in just to put them in.
Again, because every game is different for Pokemon, they can't do a region based land. Its not like Avatar, it more like Star Wars, if they recreated some place it would have to be like Batuu in Star Wars Land.
Even among the games the design of Pokemon centers, the battle arenas, etc are completely different. Like the ones in the Alola region having clothing stores attached to the pokemon center which none of the other games had.
So while you saw oh its so easy for UC to just plop a pokemon land down. I think its much farther down the land than anything else based on their own words.