Of course there is a type of writing specifically intended for students to read: textbooks.
Unfortunately, textbook publishers are in it for the money, and they know they have a captive market which is required to read their products. So they jack up the prices. More importantly, they come out with new editions every year specifically to undermine used textbook sales, even if the "new edition" is just the last one re-organized with a few new pictures and different fonts. And professors have to assign the latest edition to their students; only the latest version is in-print and reliably available, and they can't teach effectively if every student has a different version with a different chapter 5.
Because of these issues, there's just no way any university library can afford to buy them all. If we bought even one copy of every textbook required on campus, it would eat up a huge chunk of our book budgets, leaving us with a) far less money to buy actual academic books and journals (the kind needed by faculty and grad students) and b) huge collections of out-of-date textbooks that no one uses. So most academic libraries buy very few textbooks.