How are iOS apps protected from theft by other developers? An app can contain proprietary resources like a Core ML model that a developer spent a lot of time developing.

According to this answer, 
the app itself is not encrypted when it is downloaded to a device. What stops the developer of a competing app from taking the Core ML model and using it in his/her own app?
Technically - not much. Legally and practically - a lot!

You can easily copy resources such as an ML model that someone spent a lot of resources on creating.

However it is illegal to do so almost everywhere in the world, and you will be facing fines or other types of punishments if you cross that line and get caught.

Another factor is practically. Usually it is not of lasting value to have a copy of the end product. If you cannot refine it, improve on it, add to it or understand it, because you do not have all the other files, documents, and know-how that were used to create it in the first place - it’s really of limited value!