Background information (you can skip this)
There's a technology called automatic textbook billing, or "inclusive" access (IA). It is also known by many other names.
With IA, students are generally billed automatically for e-textbooks, unless they opt out.
Disadvantages of IA
Instead of using IA, faculty can adopt free online Open Educational Resource (OER) textbooks. OERs are cheaper. Also, unlike IA, OERs may improve academic outcomes. (Source.) Sadly, many faculty don't know much about OERs. (PDF source.)
Advantages of IA
IA increases publishers' revenues. This lets them produce slides, videos, test banks, and more. It also lets them produce textbooks which are far more polished than free online Open Educational Resource (OER) textbooks.
Perhaps it might be in textbook publishers' best interest to offer students extended access to their IA textbooks, longer than one year. Publishers could offer this either for no additional cost, or for an additional fee.
Long-term access might make students and instructors happier, at little or no cost to the publisher. And it might invalidate at least one common criticism of IA.
Traditional e-textbook retailers often do offer customers a choice between rental or perpetual purchase. (Example.) The perpetual purchase option may cost more, but at least it's possible.
But I'm not aware of any IA providers which offer long-term access as part of their IA programs. Why not?