Why don't we have eyes in the back of our heads? There are at least two reasons. Eyes are biologically expensive things to make and rear view (后视) eyes don't fit well into the human evolution (进化).

Even animals, such as rabbits, whose main survival advantage is rear vision, have only the side eyes. Rear vision is not so important. There are other species that have evolved this ability by moving the position of their eyes relative to the rest of their head. Many birds have their eyes positioned towards the side of their head, allowing them a very wide angle of vision and, in the case of some birds such as the woodcock, they are positioned so they can actually achieve 360- degree vision. It is also obvious that spiders, which tend to have many eyes, only have them facing forwards.

Humans, and probably some other animals, do have another adapted sense to make it up. Many experiments showed that sounds behind the head were noticed by a listener more than sounds to the front, especially if a listener was in a dark room. So, our ears can make up for the lack of rear-facing vision. You can look at it another way. Rear vision is useful only if the potential threat is visible. During human evolution, that was most often not the case.

Finally, there is often more than one potential "solution" to a given problem. In many species, the answer to maintaining 360 degree awareness of surrounding danger is to rely on many eyes. Social behavior is common in the animal kingdom and one reason for this is that there is safety in numbers. You might be looking the wrong way when a tiger approaches, but if your teammates raise the alarm this is as effective as having eyes in the back of your head.