Because the part-time volunteer contributor has already demonstrated aptitude and interest in the project, they don't even need to be interviewed or trained.

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.