Sure, I didn't mean to suggest otherwise. I just meant, if they have an endless flow of "pretty darn good" employees, they have no particular need to try to retain any given one.
Other companies may have to search hard to find excellent software engineers, but Google does not.
It's still really important. There's a cost to training and getting an engineer to understand the problems and scale that Google faces (some might come in with that, but many do not). There's also domain/google specific knowledge that's expensive to replace as well.
Yes, if someone leaves after 4 years, it's possible to hire someone else of the same ability level, but you're going to spend time and effort to make them as effective as the person who just left.
Other companies may have to search hard to find excellent software engineers, but Google does not.