I would imagine that for some games there is still a lot of money to be made making the most out of 95-year copyright lengths, and while open sourcing games per se is definitely not a relinquishment of copyright, it could affect the profitability of further monetizing those games by relinquishing some control. For example, Nintendo has re-released its retro games in various ways; I have a Super Nintendo Classic I purchased in Japan in 2017 with about 20 retro Nintendo games, and Nintendo made classic Pokémon games available on the Nintendo 3DS for some time before closing the online store for that platform.
Now, granted, there is a huge online community that does ROM hacks and disassembly, but Nintendo has been known to aggressively defend its IP (see the shutdown of the Yuzu project, for example). As a big fan of classic Pokémon games it would be cool to see a sanctioned source code release of these games, but I just don’t see it happening; there is too much money to be made re-selling classic Pokémon games in a “Disney vault” or subscription (Disney+) fashion until the copyright expires.