Why use plates?
When I was on my mission I used to be asked the question, "Why did your prophets write on plates?" The question, depending on who asked it, also was asking indirectly... "I don't see the Bible say anything about prophets writing on plates!"
The fact is writing on plates is not only unique to the Book of Mormon. Evidence of men writing on plates have been found in Asia and the Middle East. In the Book of Mormon, Jacob explains why they use plates...
That and they were also easy to hide. Mayan glyphs say a lot of stuff but most of them are post AD 400, I can imagine that most of the ones prior suffered the typical defacing that happened on Egyptian glyphs in walls of temples. Evil people can do stuff like that.
Gold was common in the Americas, so common that gold would be used like we use paper now... To keep records. Gold can last long and does not deteriorate over time like papyri would.