Why don't most animals have "heads" in the middle of their bodies?
We generally define a "head" as something that comes at the top/front of the body. I think that, we're likely to ignore examples of animals with nervous systems / sensory systems not at the top/front of their body as "animals without heads" rather than "animals whose heads are not at the top/front". For example, sponges and jellyfish.
Some animals who do, arguably, have heads, have their heads in the center of their bodies:



Image by james_michael_hill (CC BY-NC-ND 2.0)


The evolutionary - developmental roots of head position (or having a "head" at all) go very deep (although despite this, some organisms, like the starfish above, shifted evolutionarily and shift developmentally from bilateral to radial symmetry).