afaik their "average individual employee" is actually pretty damn good. Anyway that's the point, if they don't care then you shouldn't remain there trying to impress them, especially if that doesn't align with your life goals.
Other companies may have to search hard to find excellent software engineers, but Google does not.
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.