Because of affiniities. Uniformity of arms would be much weaker than mixed weapons in the land. Especially amongst trained troops. Forcing people with affinities below 50% to use spears might work against unskilled scrubs like goblin scouts. But once they face warriors they'd be at a serious disadvantage against groups of enemies trained for affinities. Having a bunch of groups trained with different weapons and armor types won't look good and it will make resupply + repair difficult due to the limited standardisation. On the flipside, they'll have far more skilled warriors than the opponents spearmen.