One guess is that the management at FB (to use your example) are more interested in acquiring the leadership component of the team than the 10 programmers. They want to hire the visionaries, the folks who did the grunt work get to come along for the ride.
That's certainly not true; entrenched players at big companies have no interest in aquiring more bigwigs to compete with. They want to a hire a bunch of staff who have produced a work sample that shows that they are a much safer bet than an equivalent number of unknown new hires.
