There's a big problem on comparing casualty rates between eras. A losing army in the 1600s could lose 70-80% of its men, since once the rout happened, many would be taken prisoners or be killed by the enemy cavalry in pursuit. Usually, only ascattered elements of the infantry and the remnants of the cavalry got away, the rest was killed or taken prisoner.
Back in the olden days a battle was a short, brutish and concentrated affair. Gettysburg was almost 200 000 men fighting for three days on maye two sqaure kilometers. With that many man on that small area, high casualties are unavoidable.
Modern war is about lines and fronts and troops are very spread out, thus casualties are much lower.