The reason for this is due to an affect that train commuters should be quite familiar with. When rain is obsered through the window of a stationary train its falls vertically, but as the train picks up speed the apparent angle at which the rain falls increases, until ultimately it is nearly horizontal. Well the same effect happens with light emitted by fast moving objects travelling at approximately the speed of light. This process known as the doppler boosting of light will cause the light to be emitted directly in front of the moving object, instead of at all directions. Hence, the light from jets is emitted only in the direction of motion, and as each jet travels in an opposing direction we only see that the jet moving towards us.