This is the usual argument you hear when it comes to old Games. It often would take way to long time to "clean" them up from 3rd party contributions/licenses etc. For a OS this is a impossible task.
Not only that but for any large company to release code somebody in engineering needs to look at it to at least know what all is there, the legal department needs to assess the risk of potential liability, then which license to release it under needs to be decided. Finally, somebody in a senior business role needs to sign off on doing it.
I think it's great when old code gets released but the reason it's rare is it gets complex and it's nobody's job to do it. That's why many such releases are thanks to an employee deciding to care about doing it and having the internal cred and sway to push it through. Or, as it appears in this case, someone who had access to the code and is awesome enough to release it anonymously years later when the business is defunct or will no longer care.
For the former, if it's really that big they'll just release it and keep a standard budget aside for if/when the unlocated people come out. The latter is what really blocks movies because it actively costs them money, so they need to be able to earn more than what it is costing them to release. This makes music-heavy/niche music films much more expensive for potential broadcasters/streamers.
It all disproportionately benefits big names and corporations while small time creators and performers get screwed.
The moment recordings came into play it all got screwed up.
Before recordings every musician made their living exclusively from performances and every musician copied each other constantly.
We lost so much with the advent of recording and the associated development of copyrights etc.
The proof is in the numbers: management and corps take the lion's share of revenues within the industry, with the actual musicians and creatives getting a paltry pittance.