Because Disney owns the rights to Pirates of the Caribbean, the characters, the locations that they created themselves, the ships and weapons designs and even the artwork.
It has to be rebuilt, from scratch, without using any of Disney's original content. Otherwise Disney would sue the life out of the developer(s). Nobody can use Disney's intellectual property and have a hope in the world that Disney won't end up owning their life. There can't be a "Captain Jack Sparrow" anywhere outside of Disney. Or a Will Turner. Or Elizabeth Swan. Or Master Gibbs. Or Tia Dalma. They also own Isla Tormenta, Rumrunner's Isle, Isla de la Porc, and on and on. That's stuff they made up and created. Nobody else can use it. Or any of the weapons they created.
Port Royal, Tortuga and Cuba for sure are fair game for anyone to use, because they really existed. Same with sloops, galleons, frigates and brigs - those all actually existed, historically.
Pantano River? Disney. Davy Jones? History. Driftwood Isle? Disney. Jolly Roger? History.
Any game developer can only use either what's historically fact, or their own creations. Otherwise Disney will own them, lock, stock and barrel. They can't make use of any of Disney's existing files at all. In a nutshell, it's theft. And Disney has long arms and powerful attorneys.