As far as games, and the broader collection of earlier closed source applications: it's incredibly difficult to clear rights if there's third-party intellectual property that was written without the intention of being open, or licensed content, etc. It also takes a bit of an army to clean/review code and comments to get them ready.
[Source: I run our open source office... we help advise internally, but aren't staffed to do nostalgia open sourcing and so need to partner with people and teams who can help]