Money, and contracts requiring us to repurchase the title after a contracted number of checkouts.

Library Nerd for 20 years here.

Physical books we typically keep between 25 to 50 checkouts. After that, we figure the taxpayers got their value out of it, and if it becomes lost or damaged, we typically write it off.

Digital content come with Digital Rights Management and contracts. That same Stephen King physical book might check out 40 times before we decide to replace it. Some of our digital content becomes inactive after 17 checkouts, and we have to "buy" a new copy to keep up with demand. It's irritating. And each vendor has their own metrics, complicating the problem.

We have our own calculations and metrics to decided how many to buy on release (X number of requests means we need Y number of copies to fulfill in Z amount of time), but digital copies are a blessing and a curse and a money suck.

Edit: That being said, nothing is stopping you from checking out books on CD, and ripping them while you browse Reddit.

What you do with the material purchased with taxpayer dollars, in your own house, is your own business as long as you do not deny access to the material to other members of th community.