Dungeons and Dragons - “The players didn’t tell us copper weapons were the cleric's one weakness!”

I struggle to understand as a GM why this is such a common problem. Bad guys don't HAVE to kill PCs, it's often not the smartest thing for them to do. They could leave them naked but unconscious, take them to be eaten later, get them in prison, or in some James Bondesque evil lair.

You don't have to be nice to them or rigg the fight (do mug/estropiate/kill one of them), but there are tons of way to create a defeat scenario where they don't all die with your campaign, it might even make for an even better story.

This is implying that GMs don't have this sociopathic tendency to want the PCs to end up as headstones. It's practically a guilty pleasure on par with Bard players trying to fuck literally every dragon they encounter.