It is very true what you are saying about the tactic employed early in the war, which did follow those of the early 1800's as practised by Napolean. However the Tactics did evolve as the war progress into more modern tactic like the use of Skirmish lines and Sharpshooting and Trench Warfare. They were even using these same tactics including the frontal attack with fixed Bayonet during WW I, 50 years later. The same problem then too, tactic did not keep up with weaponry.
The point is that it was then as it is now, that the only way to occupy territory is to put boots on the ground to push your enemy out. Hence the Bayonet Charge. But you should always look for the least expensive way in human lives cost the accomplish that purpose of occuping that territory. Hence the manuvering rather than the direct charge if you can avoid that. That too was part of Napolean's tactics.
For example, Burnside attack at Fredericksburg, VA. did not have to happen. What if Burnside had employed Hookers flanking of the Fredericksburg position in Chancellorsville? Hooker should have won Chancellorsville easily, but did not follow throught when he had the advantage.
However, in Lincoln's mind the casualities that Burnside sustained in the Battle of Fredericksburg were justifiable IF Burnside had kept on the attack. That was the basis for Lincoln's "Horrible Math" comments. I am sure that the privates in the front ranks appreciated that kind of thinking and Generalship.
The fact is that the Union soldier became higher demoralized by Fredericksburg and deserted in large numbers, some numbers estimate as high as 25,000 men left. Sometimes entire companies on picket duty would not be at their post come morning due to desertion. Hooker when he took command had to institute censorship of packages sent to the Union soldier to intercept civilian clothing being sent to the soldiers from home to aid them in deserting.
That is another story. But the application is that to maintain and army you must maintain moral. And one of the main way to maintained moral is for the soldier to believe that his sacrifice is not being wasted in vained. Hence the pointless bayonet charge would quickly break a soldiers moral if that were the only tactic used.