They are written, but not on these plates. Omni 1:18

With so many references to the other writings of the Nephites, it leads one's curiosity to want to read more.

Jarom1:14 And I, Jarom, do not write more, for the plates are small. But behold, my brethren, ye can go to the other plates of Nephi; for behold, upon them the records of our wars are engraven, according to the writings of the kings, or those which they caused to be written.

How can we go to the other plates of Nephi?   

1 Nephi 19:4 Wherefore, I, Nephi, did make a record upon the other plates, which gives an account, or which gives a greater account of the wars and contentions and destructions of my people.

Jacob 3:13 And a hundredth part of the proceedings of this people, which now began to be numerous, cannot be written upon these plates; but many of their proceedings are written upon the larger plates

2 Nephi:14 For I, Nephi, was constrained to speak unto them, according to his word; for I had spoken many things unto them, and also my father, before his death; many of which sayings are written upon mine other plates; for a more history part are written upon mine other plates.

The larger plates are not the only records that the Nephites had.

Mosiah 25:5-6 And it came to pass that Mosiah did read, and caused to be read, the records of Zeniff to his people; yea, he read the records of the people of Zeniff, from the time they left the land of Zarahemla until they returned again.    6 And he also read the account of Alma and his brethren, and all their afflictions, from the time they left the land of Zarahemla until the time they returned again.

   23 And he said: Behold it proceedeth out of the mouth of a Jew. And I, Nephi, beheld it; and he said unto me: The book that thou beholdest is a record of the Jews, which contains the covenants of the Lord, which he hath made unto the house of Israel; and it also containeth many of the prophecies of the holy prophets; and it is a record like unto the engravings which are upon the plates of brass, save there are not so many; nevertheless, they contain the covenants of the Lord, which he hath made unto the house of Israel; wherefore, they are of great worth unto the Gentiles.

According to this the plates of brass have much more scripture than the Bible.