"Why do school textbooks have large margins when the pupil is not allowed to write in the margins?"
Good question.
Books have wide margins because the publishers can use the same binding for both the teacher and student versions. In the teachers edition, there are notes, additional info, answers to questions (!), etc. in the teacher's margin.
Also, textbook publishers would love a 200-page limit. Then they would break the text into 3 or 4 volumes. When they do that, they charge a heap for the books. Example: 800-page book, $80. Four 200-page books, $35 each. And those would be paperback. For hard binding, probably $50 each. Cha-ching! I bet some legislator has contacts with the publishers. As they say, follow the $$$$$$$$$$$$$$$$.
It's NOT about kids poor backs they care about.