I read all that and still think, "you don't what you're talking about".
This is 100% a problem of dependency management, i.e. a problem common to software everywhere at all times that has never gone away. Whether more work survives is ultimately a factor of whether it's whether it's easy to make it survive, not on the "blame them for lacking effort" angle. Some folks will take the extra time, some won't.
The software that still has its dependencies, we can do something about. For most games that equates to "emulate the release platform, and we get the playable release back" - which is the defacto standard, since it's relatively easy to get copies off a ROM or CD, and it's the single artifact that is most representative of creative intent. It absolutely makes sense that almost all the effort would go towards that. We like seeing Leonardo Da Vinci's plans and studies, but he was focused on results, and we mostly know him for those results.
The source is more specialized, and not as easy to subject to the emulator model. A lot of shops "back in the day" did not even have good backup or version control practices and the game's build system involved a whole custom software project of its own, and only one person knew how to make it go. Getting it even to the point where all the code and assets were accounted for would incur a person-weeks expense, and studios then as now would finish a release with layoffs to shave off salary expense, which in turn gave an incentive to jump ship before finishing to dodge the layoff, leaving certain knowledge about the project in limbo. And employees are often not trusted enough by the management to let them bring in an external drive and dump the project: the optimism of "preserve the art for the future" goes right up against the pessimism of "i'm building a business, not art", and the psychopathy of "if they can't dump the project, they can't steal it". (yes, it happens - on both ends) There are numerous barriers in those crucial weeks around the ship date that can cut off this aspect of archival right at the start, and many a preserved game is the result of some hero who went against the direct wishes of their bosses and made copies in secret.
Nowadays, project tooling tends to fall a little more along standardized lines, and if you start with the goal of minimizing dependencies, you can feasibly have a smooth path to restoring the build without incurring the same overheads that make managers give up on archival to save a dime. So while commercial conflicts of interest remain, it's gotten easier, relatively, if your project scope is not so large that you need to bring in the custom tooling again(which, of course, is what happens in AAA). It's not "sneeze and you've preserved history" easy, but we can work on that.
And what if we're talking about an online game? That's the preservation nightmare taking place today. All the MMOs, mobile games, and so on - those are living things. You can't reconstruct a userbase. So even having running code and assets, it gives you a part of the experience, but it will be empty, fossilized. You will have to fall back on video footage to see it in contemporary context. That said there is the occasional revival effort, such as with Habitat[1].
tldr; if you want better archival, support game labor, support projects that improve basic software infrastructure.
[1] https://en.wikipedia.org/wiki/Habitat_(video_game)#Revival