Backup are not forever too... you need to check them frequently... so think about backups in some obscure room in the company for 10 years or more.
And Japan has natural disasters too...
Today backup is cheap but 10-20 years ago it was really a hell of expensive... a ZIP drive was expensive and could allow about 500MB backup... there is magnetic tape backup too.
It's a good point. We tend to think of the options available today, but in the past stuff was stored on physical media of some sort. Physical media isn't forever, nor are the machines that can access them. People used Zip drives, tape drives, even media from now defunct companies. Heck all of my graduate research work is "archived" on both optical media and Syquest EZ135 carts, and it was done this way so all my eggs weren't in one basket. Well, in my haste to get the hell out of there and to my new job, I failed to realize that the CD-ROM machine that I used to dump all my network-based files to optical couldn't handle the long UNIX filenames. It happily truncated all of them to a useless state. The Syquest carts have long since frozen up and won't spin or anything. All I have left is my resulting Postscript file to print the final document.
I'm sure there are an infinite variety of stories like this for old source code.
Could be due to copyright/licensing issues as well.
I have been part of a company acquisition. Lots of code got dumped because the licenses weren't clear or non-existent.
Sometimes, as far as the legal team is concerned, its easier/cheaper/safer to rewrite things than to deal with possible lawsuits later on.
Getting away from source code exactly and just talking about intellectual property and communication, I worked for a Fortune 500 company that literally purposefully deleted everything they could the moment it became legal to do so. Any sort of old documents and such were retained not one moment longer than necessary.
You couldn't print, backup, save old e-mails and such. You had to violate company policy to keep anything of value. They were too worried about potential lawsuits being able to use the old information.