Oral language isn't permanent; written language is.

The content of oral language is dynamic but not permanent. The advent of written language some 5,000 years ago enabled people to render language and thought into a more permanent medium - irrevocably changing the generation and transmission of knowledge. Written language, particularly after the development of the printing press, provided a lasting format and made possible the distribution of ideas on a large scale. Wide availability of printed information made it possible for communities of scholars to grow apart from each other - scholars who would then share their learning with others - leading to the development of the concept of universal literacy.