Google does not actually do much with JavaScript. Sure they have gmail, but they put a lot more effort into graceful degradation than most web companies.
Google has larger and more complex JS apps than most other companies on the planet. They do a lot with Javascript besides gmail. Maps, Docs, Photos, G+, are all large Javascript applications. And those are just a few.
These are all separate applications which sit in their own silos. They are individually complicated, but they don't need to fit into some company wide framework.
I think this is an age thing. People who grew up using the web before javascript started destroying it are more likely to accept professional responsibility for progressive enhancement, and Google skews older than many competitors.