Even on IOS, until recently, every app, as long as you allowed media viewing permissions, had access to EVERY PHOTO in your library.
To be honest it can be annoying sometimes. I was happy with the all-in photo library access restriction we've had since 2014. Any app that would do something nefarious with my pictures would have to be something photography related, in which case they can still trick me into giving full library access even with the fine-grained policies. I'd much prefer a short-lived one-time access permission instead, for those times when you need to upload something.
What Photo Picker adds is a much nicer, more intuitive UI and the ability to grant read-only access to user-selected media files. URIs obtained through Photo Picker can be persisted, but unlike with documents picker, they don't grant write access as well.
If you're curious about how the photo picker works under the hood, I wrote an article that goes into that: https://blog.esper.io/android-photo-picker-backport/
Don’t get me wrong, these are very welcome capabilities, but phones are still eons ahead of desktop OSs.
To me, limiting which folders/pieces of media an app can access seems like a win.