Last month, the Software Preservation Network (SPN) made arguments in support of a Digital Millennium Copyright Act amendment that would allow libraries to bypass digital locks on video games so that academics and credentialed researchers could study them. But the games industry is fighting back.
My guess is because old movies and new movies can be monetized just the same, but new games have all sorts of rent-extractions built in.
Also old movies tend to be worth about as much to purchase as somewhat new ones where old games are expected at steeper discounts.
Finally, an old movie takes an evening to watch, where an old game can take much longer and deliver more value
I would say it's one of two or both of the following. Firstly being that movies (and to a degree TV shows) have a hefty bit of prestige heaped on them by society, the same way that old paintings and sculptures have prestige. And that video games are still treated like children's toys by society's 'arbiters' of what is and isn't high art. A way to put it would be to compare something like the oscars (or even the emmys) to the whatever the equivalent for games have, just watching and comparing is night and day. One puts up and really cares about the aesthetic of legitimately caring about it's art form, while the other is still largely an advertisement vehicle.
The second reason is unions, essentially because if a studio decides to remaster an old movie, they may as well go out of their way to get the guy who directed it and ect. on board because they likely have to pay them anyways when it hits store shelves (for example like $1 per dvd sold, and movie ticket sold so on and so on. Though not that exact amount but you get the picture).