I have two guesses.
One, these is no big push for higher resolution displays. The two biggest desktop markets are gamers and office work. Gamers seem obsessed about Hz and that is why you see monitors with 120, 200 and even up to 300 Hz now. Enterprises are to entirely happy with giving employees crappy displays. At work last year, I was still given two crappy 1080p displays that I hated.
The second guess is the connection speed needed to drive high resolution displays. HDMI 2.0 cannot handle more than 4k 60Hz. It is only just now that we are seeing sources with HDMI 2.1 that can handle more resolution. Displayport 1.4 can handle a bit more than HDMI 2.0, but that connection is still rare specially ones that support DSC that really increases the resolution it can handle.
Like I said, I was surprised at how hard it was to setup the LG 5K for Windows. I needed two displayport outputs hooked to the add in card which is then connected to the LG 5K display. For someone who is not computer adept, I don't think they would go through so much trouble. 6K would be even harder. The 8k display from Dell also needs two displayport connections.
Apple makes it look easy to connect all their displays, but they are doing a lot of custom stuff under the hood to get it working. Step out of Apple's eco system, and hooking up all this stuff is a lot more involved.