I think android was a Desktop OS for a bit. However, Google won't because Chrome OS is their product for OS at the moment and they're planning to (or maybe already have) get android applications working on that. http://www.itworld.com/article/2854083/how-does-android-stack-up-as-a-desktop-operating-system.html
Given the ubiquity of Android, and the increasing need for fluidity between desktop and mobile, it seems that they might be better of in the long run if rather than patch the two OSs together, they started to work on Android for desktop now. Still, they have so much invested in Chrome.
I dont think theyre gonna make an Android desktop. I think they are trying to get rid of the desktop and move everything to Chrome. Chrome has drive, google docs, email I think eventually Google is going to move towards web browser as the new desktop.