In modern times, when we can so simply refer to the original documents from the comfort of our homes, via the internet, it seems “obvious” that one would refer to the original document, rather than reiterating content contained therein. But back in the day, simply referring to the contents of another document would not be considered sufficient for communicating clearly. What if you didn’t have access to a comprehensive library of previous church documents. And most of the readers of a current document could not be counted on to have easy access to such a library (heck, I’m a total pack-rat when it comes to various documents, I can’t imagine how in old days one could begin to have readily available the large number of documents you would want to have to do any serious scholarship of Church teachings).
So, for the sake of clarity, it would only make sense that you would want to clearly and completely enumerate the books of the canon, even if they had been previously stated in a document that settled the matter.
Is this really all that difficult to understand, from the simple sake of trying to ensure that this once taught, and forever held truth remains in the forefront of the minds of those reading the new documents?
And this is especially true when you realize that things required scribes to make copies, so large and easy to access libraries were not the norm. Books and letters were expensive.