I am not sure who "we" are in the question, but any vision of Chinese armies as masses of unorganized peasants is misplaced, and incorrect.
There was from at least the Qin consolidation almost always some form of professionalized military force in China. This only increased with the post-Han warring states period, which saw a blooming of military culture all over China.
With Tang consolidation we see professional military dominating both the waxing and waning periods (before and after the An Lushan rebellion). The earlier period saw centrally organized and led, powerful armies assisting in Tang expansion and consolidating Tang power, so much so that a major force was sent to subdue the Korean peninsula during Empress Wu's reign. With the rebellion, the central control evaporated, and the empire was split into rival warlord provinces, each with its own professional military.
So we are not talking chaotic irregulars, we are talking heavily drilled and disciplined armies in massive numbers, the foundation of a deep martial culture.
And by the Song, we are talking about disciplined and heavily drilled military machines, with gunpowder.