My first thought was "why don't they use Linux?".

But then I thought about it. A lot of older systems have lost graphics support in modern Mesa and perhaps the kernel. Intel also was very bad with graphics for a time (GMA 500). My brother in law found a working 32bit laptop in the dumpster. It was able to run Windows 10 without issues. It is slow, but it works and for the time being, gets security updates.

Besides appliances (factory machines, atms) and retro enthusiasts, not sure what is keeping people on 7 and 8 besides the odd application or driver.