Uhhhh
I gather you think God was too weak or disinterested to insure what He wanted in the Canon?
But he obviously used men as his instruments. Our Lord himself did not write a word, so far as we know. Paul probably dictated most of his letters.
G-d wrote the Torah Himself. It alone has never had to submit to a human authority for its inclusion in a canon. This means that the Torah is the Ultimate Revelation, higher than any other, and it simply cannot be "fulfilled" in the way chr*stians think chr*stianity "fulfills" it.
Throughout the history of Biblical Israel there were innumerable prophets. If all the prophecies had been written down from all of them there's no telling how many volumes we would have. But at a certain point in Jewish history (long before chr*stianity) the 'Anshei HaKenesset HaGedolah established a canon of Prophetic and holy writings to be publicly read and studied until the coming of Mashiach. These books they thus canonized make up the Nevi'im (Prophets) and Ketuvim (Writings) in the Hebrew Bible/"old testament" today. And some were very controversial. For example, Ezekiel was because its account of the Temple and sacrifices seemed to contradict the Torah (G-d forbid!). Similarly, of five Megillot (Song of Solomon, Ruth, Lamentations, Ecclesiastes, and Esther) only Lamentations faced no objections.
By accepting the holy books adopted by the 'Anshei HaKenesset HaGedolah one implicitly accepts the authority of the canonizing body (and this was a very prestigious body--it had many of the Prophets as its members). Similarly, by accepting the "new testament" books canonized by the ancient liturgical church, one implicitly accepts that church's authority.