Libraries could provide an unlimited number of digital copies, but publishers don't allow that unless they buy additional copies. So it's just part of the contract. If publishers allowed unlimited checkouts, then very small libraries that only have one or two copies checked out at the same time would be paying the same price as much larger libraries that may have 50 copies checked out at the same time.

If the publisher still wants to make the same amount of money even though everyone is paying the same price, small libraries would have to pay much more, and large libraries would pay much less.