Why assemble Skirmishers salvage bonus damage in opposition to Spearmen?

I’m a low degree (850) archers (steal meso civs) participant and basically feel that the steadiness between units is rather precise. I positively assemble worse in opposition to cavalry, but in finding that’s a me worry. The kindly thing that I assemble no longer realize is why skirms salvage bonus damage in opposition to spears. It seems if I create archers plus spears there shouldn’t be one unit form that can counter my army. Thanks!