Back when I was involved there was a distinct impression that people got bonuses for getting commit rights, etc - so we’d get people who would swing by making the requisite 10-15 patches - generally relatively low value changes (fixing formatting, tidying up comments, etc), get the commit rights and then often never been heard or seen from again. I didn’t know if that was just hearsay - it wasn’t the case when I was at Google but that was some time post blink - but the pattern of behaviour made it seem super plausible.