It is / can be a hardware limitation. Which is why there are advertised limits.
You mentioned Radeon. If you buy a brand new radeon desktop chipset, that does a nice big Xvimage properly. But if you go back a few years (or go down a few price ranges) you get older chips with a maximum of... drum roll... 2048x2048
It was a lot back then. I have a desktop PC with TWO video outs, and a maximum supported desktop width across those two outputs of 2048. What use is that? But when the chip was made 1024x768 was still a decent resolution and so 2048 was two whole monitors...
I don't know which NVidia driver you're looking at. It's possible (not certain) that newer NVidia hardware can do 8192x8192 but your driver doesn't expose that through an oversight or because it uses an older hardware spec. and hasn't been updated for a new one where some extra bitfields were added or a new method was used for larger textures.