This dumb question has an important answer: accountability and longevity.
It's the same reason CIOs of large companies pay more to work with companies that provide a Service Level Agreement: their bosses (aka the board of directors) wants to know that there's someone smart who will pick up the phone if there's a real problem, 24/7. And someone to sue if things go south.
If someone is making excellent OSS contributions to libraries that you rely on, it is legitimately unsane to not try hard to hire them. It means that they won't be recruited by someone smarter, distracting them from further contributions. Also, if you bring these folks into your culture, it's almost always a huge win/win because they are already up to speed without training. They are already true believers, and you're rewarding their early belief in your vision by unblocking them so that they have their needs covered, allowing them to be even more productive.
You'd be hard pressed to find a more potent way to hire top talent. The idea that you are somehow getting ahead by not hiring someone who wants to be hired is probably the most expensive wrong idea you hold.