I noticed that when talking about casualties in the Civil War the number combines all the people who were killed, wounded, or captured but for other wars, like WW2, the casualties number just includes dead, why is this?
Because it's a peculiarity of the source you're looking at?
The Wikipedia page on United States military casualties by war lists combat deaths, other deaths, and wounds for the Civil War, just like it does for other wars. (Wound figures aren't listed for the Confederacy; I'd assume that's due to a lack of data given that the Confederacy collapsed at the end of the war.) The US Department of Veterans' Affairs, from which the Wikipedia table pulls a lot of its figures, gives the same breakdown.
So there's no great question here; the issue is just why the book you're reading chooses to present it that way.