Depends on size of the project. They do it often actually. Sometimes after they have decided to move internal maintainers/teams elsewhere, or don't have budgets to support the project anymore, or the company strategy has changed etc
That's when they suddenly develop a great affection for the "community" and "giving back" :)
In large orgs it can be a costly process for large codebases. Requires a bunch of planning. Large codebases (sometimes with code from acquisitions/different source control systems/clients/sold off divisions etc) are full of all kinds of stuff (stolen code, proprietary code licensed from others, non attributed open source stuff, server passwords, user data etc etc) that no one knows anything about. All that needs to be checked sometimes by a large number of tech people and then legal teams to make sure no one gets sued.
That's when they suddenly develop a great affection for the "community" and "giving back" :)
In large orgs it can be a costly process for large codebases. Requires a bunch of planning. Large codebases (sometimes with code from acquisitions/different source control systems/clients/sold off divisions etc) are full of all kinds of stuff (stolen code, proprietary code licensed from others, non attributed open source stuff, server passwords, user data etc etc) that no one knows anything about. All that needs to be checked sometimes by a large number of tech people and then legal teams to make sure no one gets sued.