Likely because these are not end-user machines but developer machines. Most Google engineers don't checkout/write/build/test/run code on their machines (they either do these tasks through a browser or SSH to another machine).

I believe gLinux can be installed on trusted machines that you connect over SSH as well as client laptops (where you can't checkout code etc).

That said many engineers/non-engineers at Google use ChromeOS on their client laptops (same with macOS).