If you already work for them for free, why should they pay you?
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.
You get 4 hours a week for free, you make $1200 - $0 = $1200
You get 40 hours a week for $4000, you make $12000 - $4000 = $8000
By hiring them, you make more profit.
You could try to get 10 people doing 4 hours a week for free, but maybe there is a limit to the number of people willing. Let alone the coordination and efficiency advantage of dedicating more time per week to tasks.
If they don't pay you they:
- Have no incentive to make you continue contributing,
- Have no decision power over what component you contribute to,
- Have no decision power over when or how often you contribute,
- Have no ground for asking that you follow any particular process (due to the first point),
- etc.
On the face of it, however, to me at least it reads like your 'question' is less interesting: a rhetorical restatement of "companies don't do this". Then the 'evidence' you offer is an invented just-so story. Like all such stories it reflects your response to the world and the domain of your imagination, but tells us nothing about the world itself. It's a plain empirical fact that companies either do this, or they don't, and you find out which is the case by examining reality, not by weaving tales.
Telling stories to assert that the world 'must' be as we perceive it is common but without worthwhile uses outside of debate halls.
Because they'd like you to do it full-time instead of only in your spare time?
For large companies one of the obvious value-adds of OSS is it optimizes out the whole time-consuming, costly, and error-prone recruiting/hiring/training process. That all occurs voluntarily on the contributor's time who's so interested in the work they've already sought out the job, developed a rapport with the collaborators, and have been doing it for free.