Because he was most likely in a climate or situation where writing on anything but metal plates would perish, Jacob and his scribes had to take great steps to preserve a few choice things, especially for the benefit of their children. He was also probably right in thinking that his posterity would not take a lot of time reading what they wrote, and so he probably struggled to say only things that mattered most.
Book of Mormon Central, "Is the Book of Mormon Like Other Ancient Metal Documents? (Jacob 4:2)," KnoWhy 512 (April 25, 2019).