Nevertheless, I have received a commandment of the Lord that I should make these plates, for the special purpose that there should be an account engraven of the ministry of my people.
Upon the other plates should be engraven an account of the reign of the kings, and the wars and contentions of my people; wherefore these plates are for the more part of the ministry; and the other plates are for the more part of the reign of the kings and the wars and contentions of my people.
Wherefore, the Lord hath commanded me to make these plates for a wise purpose in him, which purpose I know not. 1 Nephi 9:3-5

God asked Nephi in the Book of Mormon to make two sets of plates, dividing the history of the kings (political, historical stuff) from the ministry of Nephi (the spiritual stuff).

The Bible never separates the two histories when it talks about Israel.  They are always one and the same.  The history of the Israel is the history of God.

Conveniently, the set of plates that detailed history, containing information that can actually be verified, were never found and were lost in this record.

There is no good reason why the plates would be separated unless Joseph Smith wanted to make sure no one could prove his history was wrong.

This brings up the question again, how much time would it require for someone to "engrave" brass plates?  A long time!  And remember, also, brass was not a common metal of the time.