Language has probably been with us for nearly a 100,000 years. Writing, in contrast, is at most 10,000 years old; and widespread literacy is very recent indeed. We're talking a matter of centuries.
And yet we tend to think of language through the lens writing affords. Language may be prior to and independent of writing, but our concept of language is shaped by the availability of writing as a way of representing language.
In particular, we tend to think of talk as writable. Writing, after all, is a way of recording the things we do with words.
It is salutary to be reminded that this isn't always so.
This video is painfully funny. Try to transcribe the dialog!