Google has it easy in this respect. At many other companies it can be hard to know if people are slacking off or just not that smart.
Please treat this as a genuine question - as someone who has only started working a few years ago and never had a management position, I feel my view is correct but am open to learning if/why I'm wrong.
Other companies would love to hire only Google caliber employees but there isn't enough people to hire. So you end up with a company that may have a few superstars and a lot of average programmers. If a superstar slacked off they would have the same productivity as the average coders. It is hard for a manager to complain to that superstar when his productivity is just as good as many of his peers. At Google the slacker would stand out because his peers are all superstars and get A LOT done.