Your example is a case where piracy had no profit impact, but it's not the general case.

The books are priced in part based on assumptions about how many people can read them over time. A book at a library can probably only be read by ~20 people a year. If 200 people a year want to read it, the library needs ten books. But, if we were free to post the scans online, the price per book would need to be astronomically higher.

While the usual naive extrapolations of MSRP-times-pirated-copies is also wrong, allowing people to freely and at-will render others' work unprofitable is a subversion of the purpose of copyright: To ensure that the creation of works is be profitable enough to pursue.

If someone wanted their work to be in the public domain, they could so choose.