That simply isn't true. The factions near the borders in the north and south, like Zhao, Chu, Qin, and especially Yan will have to have very different military styles and patterns from the states in the center like Song, Lu, Cai, or Cao. States near the steppe and the barbarian factions will be naturally inclined towards mounted cavalry heavy armies while the central states wouldn't have as much use for cavalry and will find infantry development more important. Furthermore, the states had very different cultures from one another, and even different outlooks.
The Qin were a harsh people who lived far to the west of where the centre of power was, they may have been descended from barbarians and so had a motivation to prove themselves, their land wasn't as agriculturally bountiful as the rest of the states, they were far from the sea, and had wide open spaces to expand to, fighting their first wars of conquest against the Shu and Ba states.
The Yan were far to the north, and their land hugged the sea. They constantly had to deal with barbarian incursions, and had to fight both the Zhao and the Qi to survive.
The Zhao were a state far to the north and west and constantly beset by barbarians, which led to the development and adoption of mounted cavalry for the first time in Chinese warfare which had previously been chariot driven.
The Chu were a very large state to the south, that had an idiosyncratic culture quite different from their northern counterparts. They were never close to the coast but thier land was very fertile, hugging the Yangtze which at this time formed the periphery of China Proper. They dealt with the equally different and idiosyncratic barbarian states of the south, like the Yue.
The Qi were a highly cultured state and among the most ancient of all of them. Their preferred military style was to play the states against one another and join on the side of the underdog to preserve the balance of power.
The Wei state is beset on all sides by enemies and in order to survive must fight hard defensively and choose its alliances carefully.
The Song, Cai, and Cao states are very small, but their populations are dense. If they can play their cards right, they can gain power at the expense of the others and supplant dominance in certain regions. For example, the Lu have the potential to overthrow the Qi if they have the right alliances.
etc.
Each faction, as you can see, has a very distinctive style and different military, sociopolitical, and economic needs to be met. They have different influences which bred distinctive unit types.
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