I guess it depends on your definition of desktop. I would call the Android home screens advanced menus. They have icons and, yea, widgets... but I certainly don't see any overlapping apps. I haven't had much use of iOS devices, and certainly not for years, but isn't the home screen on that just a great big list of installed apps... that's pretty much a menu, if you ask me.
You can have multiple apps on-screen at once. Some video players can launch a new window so you can watch a video overtop of a portion of your web browser, I use some game hacking tools that give me a toggleable overlay over the game where I have basic peek/poke and search functions, and so on.
It's just most apps don't because most apps aren't really intended for multitasking. Android started as a pretty basic phone OS and has to run on relatively-low power machines, so the people with hardware
and need to multitask on Android are relatively few.
Wii, 3DS (presumably XBOX and PS3??) all boot up into a menu. Or do you class those as desktops too?
The Wii for one has no OS, the system menu is just the default program that runs (and yes, this means that every official game has it's own copy of the Wii "home menu" built-in, it's in the SDK and Nintendo's guidelines), and it's used to boot into other programs.
The other systems have the same sort of setup, except on a technical level they actually have an OS and thus can do fancy overlay things... but they all aim for the same basic goal of having the home menu be the first thing you see, and what you return to when you kill the primary running program.
And that's how most people use a desktop. You see the desktop when you start, you launch apps, and when you close your apps you see the desktop again. Metro is another iteration of the desktop and is intended to do most of the same functions with this basic shared overview in mind, it's just a lot of the changes and design choices have been sub-par.
