It may sound trite, but the real reason is -- why would they?
The engineers are all busy working on new things. What product manager is going to say, hey, Fred, take off two weeks and take this random product from 1997 and see if it still builds at all, find the old dependencies, clean it up, talk to legal to see if we still own the rights to (x, y, z), and throw it up on GitHub -- just to expose some of our shoddy programming, last-minute hacks, vulgar comments cursing out other programmers, and maybe even some proprietary company information that we had no idea was in that code base.
There's no upside, but possibly plenty of unknown downside.
Yeah, you can see that by looking for when companies do open-source old code. It's usually a case where someone has some specific reason they want it open source, and is willing to push for it.
id Software games: John Carmack wants to get old code out there, partly so students/others have some real-world examples to peruse and hack on rather than only toy games from textbooks, and partly because he's proud of a lot of his clever hacks and eventually wants them to see the light of day.
Axiom (computer algebra system): Someone starts a proprietary software company based on academic research, and it fails. One of the original researchers has a personal interest in getting the code released, so the project can continue.
Blender: Company goes bankrupt, and the original programmers want it to be open-sourced so their work doesn't disappear. Raise $100k to buy it from the creditors in bankruptcy.
This is so, so true. So often when people ask questions like this, they seem to be overestimating the value of having access to the code that they're interested in, and underestimating the expense and risk of open sourcing it.
Very few companies would be willing to just throw the code as is to the world. Usually they're going to want to go over that old code line by line to make sure it's fit for public consumption and doesn't contain intellectual property that they don't have a right to open source. All of that stuff costs money and engineering time and probably legal time as well.
They would also have to have some reason for doing it in the first place, something more than that it could be hypothetically interesting to hypothetical people. That's a leap of faith that few companies are willing to make, and the larger the company, the more people have to make that leap and sign on to it.
Having said that, I also wish that more companies would open source their old code (for academic reasons if nothing else). I just think the reason it doesn't happen more often is for really practical and predictable reasons, and not because those companies are totally oblivious to the idea or because they're evil or anything.
Back when the idea of a corporation was first formed (I do not remember the time frame) one of the requirements to complete the process of incorporation was to prove what you were going to do for the community around you.
Companies are usually improving the community in some way (that's why they make money in the first place). Grocers sell food to the community, hardware stores sell tools and supplies for making things, even the dreaded oil companies have sold the primary source of fuel for a century (and made money hand-over-fist for it). Random acts of kindness (e.g. open sourcing projects for no reason other than "We could"), while cool, are tangential to the purpose and specialty of the company.
This is how it should be - sadly both ends (doing good and making money) are not always aligned with each other. From my personal observation I would say that they are less and less aligned the bigger the company is.
I seem to remember sending the Library of Congress a cd with source code on it when I did a copyright registration.
I guess we'll see what they've done with it when the first computer program copyrights expire in around 50 years (assuming Disney doesn't buy anymore extensions).
I think the biggest issue is that software/technology isn't "trickling down" the way that many other resources do. It's proprietary, region restricted, and short-lived.
Maybe a solution is to find some unpaid interns or volunteers, NDA them, and unleash them on the technical challenges (and scrubbing the obscenities from the comments). That leaves only the legal question for the company.
It could be a huge PR boost. EA games got some good mileage out of releasing their older titles like Tiberian Sun as freeware. Imagine if they unleashed them as open source, and thus made them into a learning tool (like a textbook) for future generations.
The engineers are all busy working on new things. What product manager is going to say, hey, Fred, take off two weeks and take this random product from 1997 and see if it still builds at all, find the old dependencies, clean it up, talk to legal to see if we still own the rights to (x, y, z), and throw it up on GitHub -- just to expose some of our shoddy programming, last-minute hacks, vulgar comments cursing out other programmers, and maybe even some proprietary company information that we had no idea was in that code base.
There's no upside, but possibly plenty of unknown downside.