Why doesn't Google make a free, compatible Windows clone, for the same reasons Microsoft as well as Google made free, compatible browser clones? I honestly don't understand it.
Don't extend your competitors platform. Chrome took IE users and made them Chrome users because Google's platform is the web, not the desktop. So Chrome has been pushing forward traditional desktop features such as offline, OpenGL, and sockets, to displace desktop software.
Look at what Apple has done with the iPad, they've been the market leader and MS has had to follow them, only recently shipping Office apps for iOS. They're blazing ahead with their own platform which serves their goals, not tired old legacy software from 30 years ago.
Also, there are countless document templates (which plague different work fields) that are utterly incompatible with Google Docs and break all over.
They might argue that we're spending more time in web applications, rather than purely in native desktop apps - This is reflected in their Chromebook offering, which largely strips away the non-web experience.