Mega-publishers are saying electronic books do not wear out, but this is not true at all. The Internet Archive processes and reprocesses the books it has digitized as new optical character recognition technologies come around, as new text understanding technologies open new analysis, as formats change from djvu to daisy to epub1 to epub2 to epub3 to pdf-a and on and on. This takes thousands of computer-months and programmer-years to do this work. This is what libraries have signed up for—our long-term custodial roles.
Also, the digital media they reside on changes, too—from Digital Linear Tape to PATA hard drives to SATA hard drives to SSDs. If we do not actively tend our digital books they become unreadable very quickly.
Then there is cataloging and metadata. If we do not keep up with the ever-changing expectations of digital learners, then our books will not be found. This is ongoing and expensive.