That explains why they are not identical, that does not explain why they are not comparable. In both cases a single bought copy is shared with an indefinite number of non-paying readers.

There are some very important differences that make it incomparable:

* libraries have a limited amount of copies. So the amount of non-paying readers is not indefinate

* the readers have to return the copies of books or DVDs they have rented

These inconveniences makes buying a copy for personal use much more attractive for the consumer.