Training. Very little training was conducted. The limited training conducted was overly focused on drill and ceremony, and admin tasks. Combat training was rare, soldiers could go months with zer0 combat training, especially in the winter.
This army had no basic combat training (BCT). So soldiers could, and did, join the army, reach a unit and conduct their first combat with no training at all.
Sidebar: I want to discuss LTC Custer when he was advance guard commander, in the 1873 and other actions. Is there an old thread explaining the 1873 campaign, to get folks up to speed on that?
Apparently Sgt Culbertson very much agrees with you as he states in his RCOI testimony the following....
"About all the instruction they had had in the duties of a soldier were what Maj Reno had given them that spring. Most of the time they were on some other duty that gave them no chance to learn how to fight".....He further states
"The recruits were brave enough but had not had the time nor opportunity to make soldiers-some of them were not fit to take into action."