Why aren't the brains of most advanced life forms in the middle of their bodies?
Because the head is where most of the sensory organs are, and once the brain started developing there it was basically impossible to move it. There is so much relying on its placement, that if it was moved many nerves would no longer be directed to the right place, as there are myoskeletal constraints around it. Basically most of an animals form around the central nervous system and moving major basal organ complexes just doesn't happen in evolution. Moving the brain would be impossible once you got to things like vertebrates. 

Smaller CPU only work in systems with lots of redundancy which is not an animal thing. Centralizing processing has it's own advantage because parts of the brain have to communicate with other parts a lot. You would basically have to have almost an entire other brain, which defeats any advantage. 

Lastly, there really is not much of an advantage in moving the brain into the chest cavity, as for most animals the brain is small enough to be just as safe in the head as anywhere else.

That said, there are animals with a brain in their midsection; they are called cephalopods.