One reason for high casualties during the Civil War (and WWI as well) is that it took time for generals to unlearn the lessons learned as young officers. Tactics which were effective against muskets turned out to be disastrous against rifled muskets.
As the historian Shelby Foote said, “The reason the casualty numbers were so horrific is because the weapons were way ahead of the tactics.”
“...that it took time for generals to unlearn the lessons learned as young officers.”
Or, the general (Kitchner) did not care much about the carnage, as in WWI, when 20,000 British soldiers were killed in one morning.
I saw an old, old WWI German machine gunner interviewed about the battle. He broke down sobbing and said he quit firing his machine gun because he got sick from killing so many British soldiers.