Most of what we have left from the ancient world is material that people felt worth copying for _centuries_ after they were written. That's a fairly amazing quality filter and gives people a skewed perspective on the overall quality of material written at the time.
A selection of work from a random library has also likely to be filtered for quality to an extent, and for the most part, the "good stuff" is going to be works that we _already_ have copies of or fragments from. Anything we don't already have a copy is is most likely going to be something that there weren't many copies made of, and usually there's a reason for that.
Which isn't to say that anything we find isn't going to be interesting for other reasons -- even bad writing is going to be incredibly useful for historical research.
https://en.wikipedia.org/wiki/Complaint_tablet_to_Ea-n%C4%81...