Libraries certainly do this. Might not be at (your suggested) 75/100 rates, but if a physical book is popular, library systems do purchase more copies.
Obviously, since patrons check it out, there is demand for it. Well, so what? There is demand for cocaine and porn; should public libraries offer that?
Demand for something doesn't ipso facto oblige a library to provide it.
Of course demand for something doesn't make it something that should be free.
On the other hand, your arguments against e-libraries are just as vague and lacking in implication.
Libraries promote literacy and/or learning with every book read. Even as a source of entertainment, it promotes a richer culture. All of which does have a total health of society benefit.
Why not? What are your goals of a public library?
Nothing with an expiring license or ongoing fees.
> Limits. You may borrow at any one time, 100 total items per library card, including up to 10 console games and up to 30 DVDs/Blu-rays.
Also, per-borrower limits are different from per-book limits.
A limit of 20 holds for a given e-book means that at most 20 people can wait in a queue for the opportunity to be the next borrower.
If a book is popular, let the patrons "buy down" their own copy, or else wait behind 90 other people.
There is always bittorrent and filez websites: the real libraries of the digital age.
There are also ... craploads of other worthwhile books you could explore that are not held.