In the 90s a lot of companies (including big ones like Square Enix) simply deleted the source code once the game launched. They truly thought they had no need for it any more.
TV shows didn't get routinely archived until quite some time after things like "video tape" existed. Now and then early episodes of something important being discovered is news.
Most of the early history of comic strips and comic books has been lost. Original art was thrown away; modern restorers have gone through some heroic efforts to piece together archives from whatever copies of the stuff they can find.
Games? Hell, there's a not-insignificant chance nobody was even using source control until the time when video game budgets started to match feature film budgets. Companies die, and if you're lucky then someone takes home source archives. before all the drives are wiped.
History repeats itself.
A good example is the Steam release of Final Fantasy VII - it actually ships the original binaries from the 90's PC version mostly intact. Code for things like the Direct3D 6 and software renderers, and MIDI sound is still there in the binary, just unused. All the new features like cloud saves, achievements, and original music are implemented using a separate DLL that hooks a lot of stuff in the game, like the graphics and audio subsystems. It's a pretty fascinating thing to reverse engineer.
That must've been an interesting job :)
There are a lot of old games that I'd like to see the code for, at least for curiosity's sake.