Even if there isn't any 3rd party code, the whole process of going through the codebase to confirm there really isn't any 3rd party code, and generally getting the legal department to sign off on it, is a lot of work in itself. My impression is that this kind of "historic source" release typically only happens if somebody sufficiently senior in the company cares enough to actively push it through. The default is that nobody does care that much, and it doesn't happen.
"Do nothing" has essentially zero downside for a big company that happens to have something of niche interest like this in its vaults.