That breaks the security promise that apps don't have direct access to other apps' data.
There's a reason you can't even access photos without an explicit user prompt.
> Not much different from the way file managers are used on traditional desktop systems
Traditional desktop systems never had heavily sandboxed apps with no outside access
No it doesn't. Giving the user the ability to get their hands on their own data is not the same as giving other apps direct access to it.
> Traditional desktop systems never had heavily sandboxed apps with no outside access
That's completely orthogonal.
Does the user have the need for that on an iPhone?
I don't think a single app on my phone has any files I'd ever need access to
> That's completely orthogonal.
Of course it's not orthogonal
You questioned whether there was a use case for file managers when apps do not share a "common file system" and are "heavily sandboxed", and stipulated that providing one would break the security promise of not giving apps "direct access to other apps' data".
Whether desktop systems have traditionally sandboxed their apps to the same degree is, in fact, orthogonal. It's a separate question entirely.
> Does the user have the need for that on an iPhone?
Sorry, but when I spot somebody moving the goalposts (and failing to acknowledge when one they staked out earlier was satisfied), my approach is not to indulge them by continuing to respond as if they're just like anyone else having a discussion in good faith. Instead, I say, "you are moving the goalposts; you are not asking in good faith". This is monumentally helpful in making economical use of my time.
Access each app's sandbox to move/copy/exfiltrate the data contained there, for whatever reason the user desires. Not much different from the way file managers are used on traditional desktop systems.