Libraries (at least in the US) have an artificial cap on the # of books they have. Which means it doesn't satisfy that "I want to read this now" craving that Netflix has. A popular book can have a multi-week wait in the SF public library.
The libraries have a license for a particular number of copies of each title that can be checked out at one time. They pay for this license, at high rates for popular books (more than hardcovers).