Because every Tom Dick and Harry might come along and claim they have God's word.
Think about the Muslims and the Koran.
Regards.
There isn't a single book of the Word that isn't intertwined with the Prophecy. It is for that reason that the Protestant Bible stands on it's own, agreed to as inspired by everyone, both in the Old and in the New Covenants.
The Prophecy proclaims the Word, and visa-versa.
It is also why I personally lend credence to several books of the Apocrypha, the Ethiopian book of Enoch, and the book of Jasher... While I will not support them as canon, as the available works are obviously not original, there is enough prophetic content to admit what once was...
Think about the Muslims and the Koran.
Anyone who has read the Koran and associated hadiths in the light (or rather, darkness) of their prophecies would not mistake them as being associated with the God of Israel. Their prophet and prophecies are directly and diametrically opposed to the God of Israel, and His Christ.