Never said that wasn't the case. Training makes a difference. Today it is discouraged to go full automatic. Also your job is probably 3 careers lumped into one.
Traning with finite resources is different when training with a huge resource base.
Today's military is thought to avoid over kill and not waste man power. In both periods mentioned we had manpower and resources. In nam for example it was common for troops to unload full automatic at a certain area hoping they hit something. Was it effective not really.
The point is the only race that even shows modern tactics is the Tau and Eldar. While everyone else have the WWII-Vietnam era train of thinking on how to conduct war. How often humanity would wipe out a planet when they could had hit the problem at the source.