We set the threshold of sensible protections provided by the app (signal) itself differently.

On desktop having a gallery app, as you say, or running an application like windirstat for example I expect the user to understand that anything stored on device can be “seen” by the app and that, if they dont trust it, having sensitive files deleted or sandboxed might be prudent. Messages are stored at least somewhat encrypted (albeit with the key in a config file) so a random (non targeted/malicious) scan would gt blobs there.

On mobile due to how opaque the os is I am thankful for the extra encyption and I would consider it a much more critical flaw. On desktop less so. Still I appreciate your point of view and a passkey to encrypt at least messages on the desktop app would be a welcome addition.