First off, WoW's graphics engine gets updated every expansion. Second, newly coded games like Skyrim are still CPU dependent. Check out the big performance jump from 3 to 4 ghz:
WoW's CPU dependence might be due to one of two things:
1) WoW tuned for casual users. Casual users tend to have systems that are CPU heavy so Blizzard allocates more load onto the CPU.
2) *conjecture, might be wrong* Think of a FPS like BF3. How many different types of character models guys are there? How many maps are there? "Not many" is the answer to both. For a GPU to render, the CPU would have to tell the GPU what polygons are visible and which polygons are in front of which. With a game like BF3, its possible to optimize these things. If you're in this room then only these polygons in the room matter. From that angle, only these polygons on this char model will be visible. Since FPS are limited in map size and character models, they might be optimized by computing a polygon display table from which the GPU can work. In contrast, a game like WoW has too many character models and too many maps such that its not possible to perform these types of optimizations without severely increasing the installation size.