Well, competition for jobs (especially the top ones) has never been so fierce. I remember my first job out of undergrad was programming for a (then) pretty well known hardware company. I cracked open a book on "how to write device drivers" the day before my interview and breezed through the interview and got the job. This would literally never happen today. Interviews (even at no-name "dog" companies) are so much tougher these days and there is so much competition that there is no longer room for mistakes.

These top employers are seen as kingmakers for engineers' careers. It's totally believable that one would spend months (or even years) preparing for a single company's interview.