There where a good number of people that could read, a smaller number that could write. Each village had a priest, the local 'head man' the more wealthy merchants, can't keep your books if you can't read. Children of even smaller landowners would have sent their boys to a school. What they wrote one is a different matter. Vellum was used many times, just scratch out the old words.Probably there just wasn't much of people who could had write and these few of documents were destroyed later either on wars or fires.