No matter what game you play, even going back to the old 16-bit 2D cards, game data always has a flow to it:
-- data gets read from the hard drive;
-- CPU processes the initial instructions, i.e. "talk to the GPU", "send this to the online server", etc.;
-- some data gets sent to the GPU for processing, some gets pre-processed by the CPU before getting sent to the GPU, & some data goes to the GPU for pre-processing before the CPU has to process it again
The GPU can be set up to handle a lot of the visual processing ("eye candy", the textures, etc.), but the CPU is still important because it handles the actual game mechanics ("player X's weapon has intersected where player Y's body is located, so execute weapon action Z", etc.). Which is why, with multiplayer mode requiring more work by the CPU, you will see FPS drops in multiplayer, even on high-end GPUs...the drops just may not be significant enough to affect the onscreen results.