Secure folder/Knox was born as an Enterprise feature to separate the work and personal use of the phone. If you would install for example some game from some shady developers and gave it permissions to read the file-system still it wouldn’t be able to grab the work documents you manage inside Knox for example.
People don’t realize how unreachable are the files until the need to access them appears. See here: Lost 1.5 Months of Notes/Where does Joplin store notes in Android File System? . No special configuration, encryption or anything (even if not stated explicitly just a normal, run of the mill Android phone). Still even THE OWNER OF THE PHONE WITH THE PHONE UNLOCKED, running anything (including the OS file manager) won’t be able to grab the files saved by Joplin.
I think a good compromise for the phones would be to enable in the app the option to use the fingerprint or whatever bio-metrics or pin the phone has. It won’t have all the usual trouble with how to make sure the user doesn’t forget the pin or how to reset it and so on (as it has to manage it for the phone already) and even if the actual security benefits would be minimal the perception boost in the eyes of people who think “this is terribly insecure, anybody can read my notes” would be huge.