re: Every nation ruled by an elected government SHOULD have a rule book
History is full of fine examples of nations ruled by the whim of whoever happens to have the biggest army. Thanks, but no thanks. At some point, to advance into real civilization, rules have to come into it that apply to everyone, including the King. That's what the Magna Carta was all about.
Oral tradition can work, but not very well for large numbers of people. The creation of written language and complex civilization at around the same time is not a coincidence.