[thrower123]

Depending on how old it is, the source code probably doesn't actually exist in a lot of cases. Game studios weren't exactly up to the latest standards of version control, by and large. Maybe there was a self-hosted CVS instance, or a dusty old version of Perforce, or a NAS that had the drives fail eons ago...

[zozbot234]

Even in that case, the copyright holder could still "release" the published binaries under a FLOSS license, and encourage the community to reverse-engineer a viable source code out of them. An overt legal endorsement of such community efforts may be just as compelling as an actual source release.