The external storage thing is an accident of history, with perhaps some MAFIAA meddling.
When the first Android device shipped, all internal storage was dedicated to OS and apps. But OEMs wanted to advertise that they had X gigs onboard. And thus they started partitioning internal storage, and mounting one part as if it was removable memory.
Problem is that apps could not go on removable memory, so people bought a device only to find that a few apps was enough to max out that side of the partition.
And thus Google came up with "move to SD".
But then that got into trouble with USB data transfers, as the "SD" had to be unmounted on the Android side before it could be mounted by the PC over the cable.
So somewhere around 3.x Google adopted MTP, and union mounts to emulate a FAT partitioned SD (for legacy reasons).
But at the same time they introduced new access permissions, but reserved write permission to actual removable storage for system level apps. Thus you could use your PC to transfer music and such onto a SD, but you could not officially do so from within Android. Various OEMs, Samsung for example, modified the permissions file so that their devices retained the old behavior.
Since then Google has in fits and starts modified the MTP support so that it can also be used by internal apps to write to removable storage without having to deal with permissions. At the same time they have tried to lock down access to the "SD" even further as various dumb app devs have used it to store sensitive user data for any other app to see.
Thing is that going via the MTP subsystem invites DRM, as the apps talk to a database that then talk to the FS. And that database can be incomplete, or the subsystem can outright lie to the apps. And it so happens that Google adopted MTP along the same time as they introduced Play Music and Play Movies.
Then again, the consumer UI/UX designers have long been tilting the windmills over the whole files and folders thing. For some reason they see it as a failure of UX, and thus something that needs to be at least hidden from the user, if not outright abolished. See numerous attempts at replacing files and folders with a grand database of metadata, that will "magically" pull out everything you have stored related to that company project or aunt Tillie as needed.