Writing things down doesn't just store and transmit information, it forces an author to crystallize and review their arguments. Writing certainly makes me more rational.

Those societies that never developed writing did have oral traditions with lots of "human knowledge and culture" but neither of these things are rational thought. Most oral traditions are rich with culture, but shot through with extreme irrationalism, exaggeration, fabuluism. This applies as much to a tribal society as to the stories your friends tell at the bar.