That is because if all the animals start growing halfly may be there will be imbalance in the biological system.
u can see the evolution of humans by an monkey that is because there was once upon a time when it had some disturbance in its growth system that is the way humans came in to picture.
I think there is no animal who r born half.... but earthworm n other organisms of such kind can live even after they r cut into different halfy.
Hope it helps u!! :) ...