Note that Google's and other American Big Tech apps do not have this issue, because Google only cares about taking permissions away from "small" players.
Some people may not want to have to reopen Nextcloud any time a new directory appears on their device so they can add it to Nextcloud, while the other American bigtech backup apps can just pick it up automatically no problem.
Spotify did this all the time where they would complain about Apple not allowing them access to some private API and then when they did didn't even bother to use it.
Do you really think it seems unfair that a file sync app would want to access files?
If you're thinking of another API, they support an additional file access api that allows selecting individual files, not entire folders. This is also not what users expect.
Scoped storage via SAF allows directory tree selection.