This is unrelated to Full Disk Access, though.

There are already folder-specific permissions for every app, including non-sandboxed apps: Desktop, Documents, Downloads. FDA is "everything else". The user has to specifically grant each of those permissions via a system dialog.

With sandboxed apps, you grant access to a file outside the sandbox via a system dialog, open or save. But with non-sandboxed apps, if there were separate permissions for each specific folder, there would have to be separate permission dialogs for each of those folders, and then macOS would become even more of a permissions dialog hell than it already is.