Estimates of how much of gaming history is commercially available in any legitimate form put the figure low – in the region of a tenth for titles more than a couple of decades old. The rest is out of print, delisted, or tied to hardware and services that no longer function.
Why games disappear
Licensing, mostly. Games containing licensed music, sports likenesses, or film properties have rights that expire. When they do, the publisher cannot sell the game and often cannot legally give it away either.
Corporate history is the other reason. Studios close, catalogues are sold, and after a few transfers nobody is certain who owns a title from 1998. Establishing that costs more than any plausible revenue from re-releasing it.
And storefronts close. Games bought digitally on a platform that shuts down are gone, including for the people who paid.
What emulation provides
Hardware behaviour reproduced in software, so a game written for a machine that no longer exists still runs. That is the entire preservation mechanism for large parts of the medium.
The legal position is genuinely mixed. Emulator software itself has been found lawful. Distributing game files generally has not, and the distinction leaves preservation work in a permanent grey zone.
The libraries and archives problem
Institutions that would preserve games properly are constrained by rules written for physical media. Providing remote access to a preserved title is typically not permitted, so a game can be archived but only played on site.
Researchers have argued for exemptions for years, with slow and partial progress.
What publishers could do
Release source or binaries for titles they no longer sell, as several have. It costs almost nothing, generates goodwill, and preserves the work their own studios made.
Most do not, because the legal review costs more than the goodwill is worth on a spreadsheet.