You know what every one of those things says in common? every generation between when it was created and now has decided it was important and made effort to preserve it. We have no clue or maybe only slight clues of what was lost because at some point some generation decided it wasn't worth preserving. Sometimes it was a hard decision for that generation when they realized they couldn't preserve everything, other times it was not cared about at all. Either way though, it was lost for all time because the chain of caring enough to preserve it was lost.