The basic reason is that no two riflemen see and perceive exactly the same. Small individual variation in shooters' individual concepts of "correct" sight picture and alignment mean that the same rifle will shoot to divergent points when aimed and fired by different folks.
https://www.milsurps.com/images/impo...7/scfhuo-1.jpg
If soldiers simply made corrections with the rear sight, its range markings might no longer be useful in a combat situation where troops estimate or are given the range of targets on which to fire.