[aamar]

Source code is intentionally kept secret and is different from the widely-distributed version.

The book-equivalent of source code is the collection of discarded drafts and working notes of the author. These, too, are often lost, but that has little to do with whether they are digital or not.

It's expensive to properly store and maintain physical copies of things like books. I understand that archivists have some trepidation replacing a known system of storing physical books with a relatively unfamiliar and untested system of storing digital goods. But the bugs are already being worked out; if you want to make sure something lasts, make it digital and make sure lots of others can get to it.