Tacking on: as far as translation of ancient texts is concerned there is also a selection bias. It is far more likely that an important formal document endured the times than some every day scribble. Of course a political treaty is crafted, conserved and replicated more carefully than a note someone left for their neighbor. Both the skill of writing and the materials required were much rarer and access more prevalent among the upper classes. Finally important formal documents are more likely to be translated precisely because they are important. Imagine that in 2000 years from now you would be one of the few scholars capable of translating English. You would be much more to likely to study and translate the declaration of independence than some mundane Twitter post.