The problem is if you put everything in a 13Gb update every time they release a World Update, those with an SSD without unlimited storage might start opening threads to complain about their valuable space being occupated with regions they are not gonna use.
I know of people that only fly Europe/Asia, so they do not install the USA update for example.
Having everything in parts allows you to customize what you want in your sim. It is modular and works really well imo.