Unlike physical books, the First sale doctrine doesn't apply to e-book licenses. Typically, the license expires after a couple of reads, and the rights holders has a say on how many times a copy may be lent out, and how many copies can be borrowed simultaneously using DRM - which means they can set the average price paid each time an e-book is borrowed.

I listened to a fascinating podcast on the history of Overdrive / Libby a few months back; might have been produced by NPR, but I sadly can't find it.