Perhaps I wasn't clear. There are two aspects, there is 1) the storage box and 2) the encryption. A safe is just a storage box that is difficult to open. That difficulty is the only security it provides. Any documents inside that are not encrypted rely on the security of a difficult to open ~500 kg box that is bolted to the floor. If it isn't secured in place then it offers no protection at all, except maybe to fire, which is the main difference from a mobile electronic device. An additional layer of security can be obtained by encrypting the documents inside. The key/combination to the safe is protected by the 5th amendment as is the encryption key to the documents so there are potentially two layers of security.
Mobile electronic devices don't have two layers. They rely only on access to the encryption key provided by the password, finger, etc. Yes, for the pedants, one could employ a separate lock to see the file system and encrypt the individual documents on the device or lockout the i/o or other contrived method to more closely match the safe paradigm but then it quickly becomes impractical since the device gets so difficult to access it then offers no benefit to it being mobile. After all, safes are largely safe because they aren't mobile.
Besides, at some point it becomes pixies on the head of a pin supported by encryption turtles all the way down and where do you draw the 5A line? It only makes sense to draw the line at the beginning or not have a line at all because an arbitrary line will always be arbitrary.