Actually, a huge number of those same users have been basically baffled by file managers for decades. I've supported any number of family, friends and acquaintances who have been using computers for decades who never really 'get' the file system organization. They may figure it out by doing things by rote - that is, they follow exactly the same steps every time - but if they miss a step, or accidentally save a file in the wrong place? They can't figure it out. I've seen this across WAY too many people in too many lines of work - people who are smart and successful in their own lives to think this is a quirk.
It was GENIUS to leave the file system inaccessible to the users in iOS. Want a file system? Choose a different device.
I'd say that is partially because before Win 7, the Windows file manager was a ghastly piece of ****. It was no good for the power user and could be confusing for the average one. Finder is still a terrible file manager in general.
There is nothing genius about leaving a file manager out. It would've been a far better idea to provide an extremely simplified one, with only the simplest actions available, tailored to the far more limited file management needed on mobile devices. People are getting along just fine with the Photos picture gallery, I don't see why this would be any more difficult if something similar was for other filetypes. Right now iOS is generally either "here, these are the files you used with this program" or "nope, doesn't exist" if you try to access the same stuff in some other program.
Right now Apple seems to do bare minimum when it comes to tailoring iOS for the iPad. It comes down to apps themselves to provide iPad specific functionality (and that is why I'd pick the iPad over Android tablets right now) while the rest of the OS is same as the phone version but with a few added gestures.