The reason is training. You can't train a mass conscript army that well in the time available during a large war like WW2, and then the training becomes less efficient in making people kill.
It's quite simply a question of human psychology. What the modern armed forces do, is, in effect, a small form of brain washing. It's efficiant, too.
That is part of the reason we have guns. Having guns lets one learn the basics of using them and translates over to other weapons. During WW2 the English had to teach all of the troops how to use a gun because nobody has them and less time to teach other things.