Most people who create a wallpaper tend to do it in the res that they use.
Of course it depends on the type of wall...but a full scene panoramic type of wall is impossible for someone using 1024x768 res. They just can't visualize something on their monitor to create a 1920x1200 wall.
Sure, they can make an abstract in Bryce or C4D in a larger res and it will probably look ok if they crop it right, but for the most part, a really nice looking wall in a high res needs to be created by someone with a large monitor, and sized down, not the other way around.
24" plus monitors are becoming more popular so there will be more walls available for them, but they are still in the minority.
Sites like InterfaceLift tend to have a fair amount of quality walls in larger res. Check around, it takes some time right to find good hi res walls.