A physical desktop is somewhere you lay out objects and organize work, and desktop GUIs have metaphors for that (windows, icons). A phone is more like a pocket notebook, a camera, a map/compass, etc. You can take it out of your pocket on the subway, in a park, on a date, or during a hike; you turn pages to find the information you need, point and shoot a photo, figure out where you are, write down a phone number, etc. There's very little in common with how you'd use a physical desktop, and early smartphones (pre-2007) that tried too hard to be like a desktop really, really sucked.