Because generally, the rich are the ones who miss out in the former, while the poor miss out on the latter. Someone whose house gets destroyed will lose out a lot more from that, then from some small fraction of their pension fund going belly up because the belligerent nation has stopped making bond payments.

Since wars tend to be started by the rich (and their proxies), and fought by the poor, this puts skin in their game.

At least, that's the reasoning. I don't really agree with it.