Chromium is open source, and will run try jobs for anyone from anywhere around the world (we won't accept your commit without lots of checks, but we'll run your tests). That means that we don't get to rely on a lot of Google-internal technology, including our production data centers.
Also, we are compiling and testing on Mac, Windows, Linux, Android, iOS, ChromeOS, and historical versions of all of the above. That's something that a mostly linux-server-based company doesn't do a lot of. So we maintain our own datacenters with our own hardware, and don't get to take advantage of all of Google's scale.