It's not about amount of discs. It's about dev time. If it was one whole game they'd not only bloat the budget to dangerous levels, it'd likely be outdated by the time it's finished and will either need to redo large parts of the game so it looks modern or release a game that looks like it should've released 5+ years ago.

Imagine if FFVII didn't split up it's remake, not only would we not have any of it right now. When the third part comes out it'll all just look like a PS4 game on a PS6.

If you want to argue whether or not they should go balls to wall that requires them to make a series of games so large is infeasible as a single entry, and do something a bit more budget friendly like the original, then fair enough. But then it'd just be a remaster or something not that far removed.