Reading and writing are enhanced language skills that can be learned after that. Humans are capable of teaching themselves to read (with difficulty, but it can be done - just as most people can learn the basics of most languages, given tapes and books.)

The point isn't that language exists, but that enhanced skills allow information to persist and accumulate outside of human brains, and to be transmitted without personal contact.

And it's not just writing. Some of the most popular language courses are spoken-word. They're recorded and replayed to order.

Again, the key difference is that they continue to exist independent of direct personal contact. Just like iPad games for kids that teach them word basics when their parents aren't around.