Their job is to make laws. It is not possible except in some simple cases to make those laws completely specify every detail and cover every case when dealing

Their job is to make laws. It is not possible except in some simple cases to make those laws completely specify every detail and cover every case when dealing with large complex topics, so there will almost always be cases where those executing the law will need to have some gaps filled in order to carry the law. That was true long before Chevron, true during Chevron, and is still true.