Open source has been a boon for tech monopolies. If you're a long-lived code monopoly, you dread waking up one day to find your code is in an obsolete language and you can't hire people to work on it. By open-sourcing your code, you can ensure there will always be people who can work on it. In fact, they will voluntarily train themselves on your code before you even meet them! They'll contribute bug fixes to it for free! You only have to pay a handful of maintainers to control the project. All the while, you'll earn goodwill for 'supporting open source' and 'not being evil'.
In the '90s, people controlled their data and complained they didn't control their OS. Today, we hardly think about the OS because we've lost control of our data.