I'm not talking about products like the first iPhone, I'm talking about products (like gameboys) that are still cool and interesting, but completely irrelevant for the company that invented them.
Why don't these companies open-source their old products?
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.