This is not true. I use git practically every day, working within the main source tree. Google's a big company with thousands of engineers and multiple ways of doing things. Some use git, some don't. You use what works best for you.

Care to elaborate? I'm curious to hear why this might be.

Not true, a lot of people, including me, use git at Google. It's not a default options, but you can use it if you want to.