The short answer is that copyright holders want to make money off of the books they own the rights to, and because of that they sell the license to their digital books to libraries under strict terms.

The longer answer involves explanation that not only are libraries limited in the amount of simultaneous copies that can be checked out, but they are also limited in the number of times those copies can be checked out before the library has to renew the license.

The whole issue is very complicated, trying to balance the rights of authors and publishers, with the public interest, and many misconceptions about copyright and economics on all sides.