>>8120652Most likely it's because most computer users appear to have gone directly from using one or more 1080p HD screens straight to using one or more of the latest (curved) ultrawide screens, and so they completely skipped using a regular 4K screen altogether.
That and the 4K resolution mostly seems to be used for television viewing and games consoles with much larger screen sizes than you'd normally find sitting on a computer desk.
Have you tried searching for "computer wallpaper 11520x2160", this seems to bring up multiple results for wallpaper sites offering at least some papes in those dimensions.
Or maybe check out a commonly used site like
wallpaperswide.com and search for either UHD or 3xUHD papes on there.