Perhaps there’s a point at which people will do it solely for the money? (And once the money is no longer an issue, they have nothing left to stay for?)
Google has three things that attract talent — a) money and perks, b) resume fodder, and c) some interesting problems to solve.
However, it doesn’t take long for B to become moot (it only takes a couple of months to be able to say “yeah, I worked at Google”). A and C are ongoing, but there are points at which neither of those things can overcome actual culture.
I don’t know as much about Google’s culture as I do of some of the other SV companies, but I’d wager that it’s demanding and there’s a high rate of burnout, particularly in the more prestigious positions. It’s great while it lasts, basically.
At some point, the project(s), money, and other perks won’t be enough to overcome the cracks in the culture, and the fact that some of those employees are calling their savings “f-you money” in the context of Google suggests that there are some culture issues that are pretty glaring to an insider.
Many people will stay in a horrible job solely because it pays slightly above market average. They don’t want to risk that little bit of extra money on the possibility of a job that’s better for their mental health, because they fear the possibility of the new job being just as bad. At some point, though, income exceeds expenses, and a money-savvy person (or not-so-money-savvy, if the company pays way over) can build a savings that allows them to seek out the better company without the risk and fear of homelessness looming over their head if finding that better company takes more than a couple of weeks.
If you’re going to pay your people exorbitant amounts of money, you probably want to make sure your culture encourages people to want to work there.
tl;dr — it’s the culture, stupid.