Why do we need a monster computer? It's quite simple. The PC does not have the hardware that consoles have. So, in short, the PC hardware cannot execute the things the console hardware does. So the processor must take the load and process this information. Mostly only the gfx is pushed onto the gfx card (after being translated), everything else is handled by the processor which is WHY we need such monstrous machines.
Also take heed that windows is a multi-threaded OS. Just running the OS spawns about 300 threads which the cpu power is divided within. No such thing exists on consoles. I'm sure there are things I don't know about consoles, but this is the basic of the problem I think...