Because there's no such thing, when we talk about bottlenecks it's referred to the cpu.
A 480 gtx doesn't restrain an i5-3570k outside gaming, the question should be if the 480 gtx is still a valid option for gaming or there're better cards out there.
Huh no such thing as a GPU bottleneck? That is news to me! And who are these "we" that only use bottlenecks to refer to the cpu?
And yes you could get more performance out of your CPU if you upgraded your GPU.