1) Larger companies get away with inefficient recruiting because they can. If the same company abuses you twice, and you go back a third time, what's that say about you? It certainly goes a long way to explain why they can do it.
2) If you're considering going back on an offer, or interviewing just days after starting a new job, you don't have much room to complain about bad behavior on the company's part.
Perhaps another reason that Google contacts many people the same time is their heuristics. They have analyzed what good potential hires look like, and instruct their recruiters (who all use the same few online resources) to screen for it.