Scaling makes them different. A physical book can have but one reader at a time, and it can only be lent a limited number of times before wearing out, all of which limited the impact.
That limited number of times is probably quite high though. In our college library there were regularly used lend-able books from the 1920s/50s and older which hadn't worn out. Authors probably shouldn't hold out too much hope of repeat sales to a library due to their books wearing out!
