In a party, you're directly responsible as part of a team. There really isn't too much sense of required teamwork in campaign - most everyone's there solo. Yeah, you might help someone out because if they die, the mob will come eat *you* next... but *your* getting exp is not directly contingent on *their* getting exp, like it is in a party.
Besides, if you let people die, then you get to have the fun and exp of raising them while you're waiting for the next wave of mobs to arrive. Melee exp *and* raise exp, have your cake and eat it too! See, there are rewards for being devious, unreliable, and someone to be avoided.
