The Players... well, to put it simply do not work well with others. Hell, lets not beat around the bush, their homicidal maniacs. They keep trying to kill each other; I say trying because none of them seem to know how to stay dead. Maybe they're all immortal, maybe they're just a bad continuity, maybe Tim just doesn't want to invent new characters every time he makes a murder joke. Probably a combination of the last and one of the first two.
There are a lot reasons why they are like this, probably, but the end all reason is that they each suffer from their own unique brand of insane. You can read about them on their individual profiles.
As you know, these aren't the only characters in CAD. Be sure to check out the Main & Side Characters as well.