Why virtual desktops always have same background?

Virtual desktops are here for a long time, and I never got comfortable with them. One of the main reasons that it takes effort to think and assign different desktops for different workflows/situations, and when you do that - it is still not easy to track in which desktop you are at the moment. Usually you have to look for some little number or highlighted square somewher in the taskbar, which is a distraction.

For a "space" to be identified as particular / different, it should be easily recognizable. A different background or even colour scheme is an obvious solution. But in practice most virtual desktops on Windows and on Linux don't even have a possibility to have a different backgrounds, not even speaking of other differences. Why?