Why can't books have these sorts of margins anymore?

Today's #ManuscriptOfTheDay is Ms. Codex 1243, a 13th c. copy of parts of Priscian's Institutiones grammaticae, also known as De constructione, a guide to Latin grammar popular in Medieval Europe. There are extensive annotations, as here on f. 5v

Online: https://t.co/2kKngcZFHr pic.twitter.com/CMfEtRhmn0
— Schoenberg Institute for Manuscript Studies (@sims_mss) December 15, 2021