Why can hearing loss never be fully restored?

Actually, it can be: In some cases, full restoration of hearing is possible, but you are right in that it often can not be "fully" restored. If the nerves that carry sound from the ear to the brain are damaged, we do not have the ability to fix that, although progress is being made on new technologies, including cochlear transplants. If it is damage to the little bones in the ear, hearing can be greatly improved.