if you ask Knives, they're just not dying fast enough.

Plus, when they kill each other, Vash really can't hold himself viable for that. So Knives likes to cause all sorts of chaos [i]himself[/i] so that Vash [i]does[/i] have something to blame himself for, cuz he kind of likes to torment him.

Plus they did manage to last like 100 years, and I think their numbers grew in that span of time since there weren't that many survivors initially.