The video card doesn't know anything about the game you are playing.. Where the trees are, who is shooting at you, what is gravity (well maybe these days they know), what color things should be, and basically WTF are you asking it to do?
The CPU and the rest of your system use the graphics card to draw all that stuff, but something still has to create it, and tell the GPU what is going on.
Most of that stuff is constant, which means that the CPU has a 'fixed' load on it, to some degree. That's why raising the resolution or rendering detail increases the load on the GPU compared to CPU..
If that doesn't make sense, let me know and I'll try again..