Because the processor in the DX80 is very basic and isn't capable. It can't even do a simple Multi-site call, so being able to do more complex tasks like the image processing involved in a virtual background is beyond what it is able to do.
The newer Webex Desk Pro is built with similar hardware to the rest of the newer Room Series endpoints and is significantly more powerful, so that should be the endpoint to chose if you want a desk based one that is capable of more advanced video processing.
I have registered a Webex Desk Pro through the expressways via MRA. I use the virtual background feature, it works fine. I am able to select one of 3 default images or laptop wallpaper or blur. I am not sure If I can use my own image.
There is the ability to share your laptop as a background, and now about seven custom virtual backgrounds total (I’m on beta firmware so you could be a release away). There’s also camera control which I believe is new.
I think soon custom virtual backgrounds will be the next thing available, it is important admins can publish these out centrally.