Moving something from a datacenter to a cloud provider isn't something you "just" do. There are tons of variables involved, and shifting projects between these two types of platforms is a full time job for full teams of people. Compounding this, FFXIV wasn't designed to run on a public cloud provider. Their backend code isn't organized in a way that AWS or Azure expect things to work. Services don't communicate with cloud servers in mind, etc. It's like going to Chicago and expecting a map of New York to tell you where everything is.

I do this for a living; moving entire projects into the cloud takes months to years for *well* architected projects. I have no doubt they looked at what it'd take to move XIV's ancient spaghetti code onto AWS and concluded it would be easier to just make a new MMO designed on scaling cloud servers from scratch.