Good question. I hadn't thought about that. My only thought was that an extension seems too cpu-intensive.
So this feature is available on all platforms except Chrome OS. For all other platforms I can either download software (Mac, Windows) or this feature is built into the OS (Ubuntu).
From a user point of view, I think Chrome OS would make more sense. If a user wants this feature, they want it for the whole OS, not just the Chrome browser.