“Garbage books” are mass-printed, but aren’t mass-written in a mass production sense. Mass production is about producing fairly exact copies of something that was designed once. The design part has always remained more artisanal than industrial. It’s only the production based on the design (or manuscript) that is industrial.

The difference with software is that software is design all the way down. It only needs to be written once, similar to how a mass-produced item needs only be designed once. The copying that corresponds to mass production is the deployment and execution of the software, not the writing of it.