You didn't understand me. Computers need electricity, when that electricity is switched off, the entire collection of works becomes inaccessible. This can be a short-term outage or it can be a longer one in case of e.g. a natural disaster.

And it's not true that "ebooks would/will never just disappear"; if you don't proactively care for the health of your storage media, they will fail; from an archival point of view most storage media will almost certainly fail in short order (years, decades at most). I have piles of CDs, floppy disks and hard disks lying around but I am not confident I can access all the data. Rather, I'm sure only a fraction of the files will still be readable.