An app can ask you, as user, to pick a file for reading/writing/creation, and it receives access to something very much like a file, but the app isn't even permitted to know whether that's a file on the local device or an on-demand download/upload by another app.
No, you can't see your file system. Not yours.
Edit: There's a load of suggestions downthread, I think I might be on an older version of ES, maybe pihole is blocking some of the ads others are complaining of.
Regardless of whether you think it's good or bad design, at least it's consistent IMO.
So is this to help protect apps from each other? And/or to protect users from malicious apps?
And what if you've rooted the device, and installed your own file-management app. Could that app manage files normally?
One angle is it prevents apps from scanning your local storage for files and stealing them.
Another angle is that it blurs the line between cloud and local so that storing data on cloud services gets treated the same as local data. If you happen to own said cloud storage services (cough Google cough) then that direction aligns with your corporate interests.
I think you can manage files even without rooting. I plugged a phone (XZ1C, Android 9) into USB now, the computer offered to open the phone's storage, 2-3 clicks later and I see the photos that are stored on the phone.
... and which is also completely unsuitable for content that implicitly depends on multiple files, e.g. locally stored HTML files: For the case where an app is launched because you wanted to view a particular file, you'll only get access to that "something very much like a file" for that exact file only, but you (respectively your browser) cannot access any other associated files that might be required (images, style sheets, scripts and whatnot) that way, and neither would navigation among separate files stored locally work.
But without an app to access it, how will the user get to the files?