1. Google's tech staffers are supposed to be some of the top people in the world, and Google spends a considerable amount of resources hiring them. It seems foolish not to keep them on the bench a bit, unless Google foresees that it will be shrinking soon.
2. The people not laid off will very much notice their colleagues being zapped the moment they're not needed. They will rationally tilt towards leaving as well, if they get any whiff that they might be next. It's far better to leave first than to be laid off, if you're sharp.