Because if good games from a decade ago are freely available, they can't shove a new overexploiting live service game down our throats when it pales in comparison to the entertainment that's available for free.
They can only sell less for more, by taking the previous option off the table.
The same reason a movie theater owner can't show Pee Wee's Big Adventure every weekend. Value is derived from exclusivity. Exercising your "rights" to a work means preventing anyone from having access to the work unless you are paid when and how you want.
I would be all in favor of "Use it or lose it" rights to Digital Distribution.... Don't offer a reasonable way to access a product? Can't bitch when Abandonware sites give it away for nothing.
Copyright is not "use it or lose it", but as it is, it is unworkable for digital media. Computer hardware doesn't last a century and with no other measures being taken to preserve that content, it's effectively doomed by the law. It also doesn't reflect a world where average people make edits of copyrighted content as a means of expression without seeing any problem with that.