Signal could add app-level encryption, but who would this serve? Signal can't do anything better than what the OS/hardware provides in terms of encryption. Even if they let you specify your own signal-specific password/encryption key:

* Non-technical users either won't use it, or will use a weak key

* Technical users are better served by making sure their device is secure and hard-locked with a strong passcode (tip: 5 presses of the lock button on iPhone wipes in-memory encryption keys, essentially exiting "AFU mode")