Yes, I think, the explanation is the difficulty level. On hard +4 attack bonus to the AI side (on very-hard +7 attack bonus). On very hard, a gaul warband/chosen sword-men charge would send my hastati routing in no time. On normal difficulty, my hastati would throw their pila, the lines would clash, the second line would throw their pila into the thick of it (killing off some of my own troops) and the barbarians would rout on the sight of my general flanking their army.

stretching out the roman battle-line does not solve the topic issue: the author wants to fight in the historical roman way... in rtw, it does not really work :(