Much of the disparity was due to the state of medical technology. Many of the wounded who died in the War Between the States would have been saved by WWII medical technology and converesely the numbers of WWII wounded would have been VASTLY higher with Civil War medicine. A more useful ratio is wounded vs dead, far higher in WWII. 96% of the wounded who made it alive to Battalion Aid and higher medical facilities in WWII survived.
They hadn’t even discovered (Not til the 1870’s) Joseph Lister’s germ theory which was instrumental in developing practical applications of of disease control (notably gangrene) with respect to surgical techniques.
What you say is true but... the lions share of the massive casualties had to do with a basic mis-match between weapons systems and infantry tactics in the 1860s battle space.
In other words, if you lined men up in ranks and files today and marched them around in formations, and stood them up in front of the opfor, they would be mowed down the same way they were then. The rifled musket with the minie ball was so much deadlier, more accurate, and at greater ranges than before that the kill ratios were immense.
The solution was trench warfare which both sides adopted by Petersburg. The massed formation was never again used by a major army after that. The development of the gas operated machine gun sealed the deal - by WWI there was no way to get out of the trench without being cut to pieces.
This stalemate remained until WWII when air power, read dive bombers, could be used to blow the enemy out of their trenches and tanks could roll across the trenches, the barbed wire, and no mans land.
Oh I agree that medical tech made a huge difference. However, given the vastly larger scale and much higher lethality of the weapons used in WWII, the closeness of the casualty count (CW was actually higher as someone correctly pointed out) is a testament to how deadly the Civil War was even given their far poorer medical tech.
Largely because of parity in weaponry and military tech and training, not to mention strength of ideological motivation, civil wars are almost always disproportionately deadly. Add to that the fact that almost all of the figthing occurs on one side’s or the other’s home territory, they are true nightmares to be avoided if at all possible. Sadly, sometimes it is not possible and even the nightmare is better than the alternatives.