It appears that the record being kept by Laban was not finished or complete. It was a work in progress. It was being added to as part of an ongoing process. Unlike our Old Testament, the brass plates were not a treasured narrative of events that took place thousands of years before. They contained current words of Jeremiah, a contemporary of Lehi.
This set of plates was likely a sacred temple or royal record. Since this collection contained all five books of Moses, it must have included the book of Deuteronomy. Most scholars believe that the book of Deuteronomy had gone missing for a long period of time and that it was King Josiah who found the Book of the Law when they were cleaning the temple in 628 BC. This resulted in the major reform by King Josiah in 625 B.C., which consolidated all of the worship of Jehovah in the temple of Jerusalem. Lehi would have been well aware of this reformation. We do not know whether Lehi agreed with all of Josiah’s changes. There were things about the Josiah reform that the prophets in Jerusalem did not like. However, there were other parts of the reform that were accepted by all.
Soon after the finding the lost record of Deuteronomy and in conjunction with implementing his reforms, it is likely that King Josiah would have made considerable resources available to make a more permanent set of metal plates with the now complete scriptural record (including the newly-found book of Deuteronomy) so that it would not go missing again. He also would have wanted to be sure that the book of Deuteronomy was firmly established as authoritative.
Regarding ancient records in general, modern people must consider that ancient Israel was a very different world than ours today. Anciently, how could a person even get a copy of the scriptures? You could not simply go to a store and buy it. There were no printing presses at the time. Even if you only wanted a hand-written copy of the scriptures, how would you go about getting or producing it? Acquiring writing materials would have been difficult. One Dead Sea Scroll of Isaiah is 26 feet long. That is a lot of leather. How many ibex hides would it take to make that much parchment? You would then need a master copy. Acquiring a scriptural record would have been an expensive item. Finally, in the ancient world, only a few individuals knew how to read and write. Someone would have to copy the scriptural record word by word. It would have been tedious and labor-intensive to hand-write the entire record. You could pay someone to copy a set of scriptures for you, but it would have been a big job. It would be costly. You get the picture. These plates of brass would have been very rare and very expensive.