Earlier quoted context omitted.
> If you already work for them for free, why should they pay you? Because they'd like you to do it full-time instead of only in your spare time?
But why not hire someone else to do it full time so you have full time + part 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.