Why a brain's hemisphere controls the opposite side of the body?
"How" is easy to answer (signals from the right side of the body cross the midline and end up in the left cerebral hemisphere; answered well in a previous question). This pattern of wiring appears to have existed since bilateral symmetry first evolved.

"Why" is not a well-defined question; the possible evolutionary advantage gained by having cross-wiring is unclear.

To answer your explicit question: there is no "opposition" in any meaningful sense between the location of a sensory nerve on the body and the location at which the information is processed in the brain. There are a raft of visual areas in the neocortex, spanning from the back but extending forward. The areas that control eye movement are much more frontal ("frontal eye fields"). Before arriving in the neocortex, signals from the eyes pass through the dLGN, a nucleus in the thalamus which is a subcortical structure closer to the middle of the brain than the rear. This flow of visual information is similar across many (most? all?) mammals.