I think a lot of it stems from an attempt at simplicity in design. At the time, Windows Mobile was offering a fully customizable filesystem, but Apple wanted to go the route of simplicity when they introduced Apps with iOS2. You tap on them and they work, delete them when you don't want them, etc. Everything is so isolated as a result, and combined with the lack of any sort of Downloads or file creation in the traditional sense, it sort of makes you feel like the data of your apps is stored in really remote, weird locations you can't ever access.

Simplicity like that was part of what made people think not just of productivity but also of fun when they looked at a smartphone imo. Still not very fun to deal with. (Never write a document on your phone. It's not worth it)