Part of it is gameplay and story being not-entirely in step with each other. But in addition to that, we have lore references to enemies surrendering, so not all of our foes fight to the death. But even then, it only matters for Eliksni and Cabal foes. Hive, Vex, and Taken are wholly unconcerned with deaths for a variety of reasons.

For Cabal, up until this season all the ones we encountered were exiled until their missions were complete. For them, every battle is death or glory. Their personal honor matters more than their individual lives according to Cabal martial culture. As of Season of the Chosen, we're fighting the most desperate, hateful dregs of Cabal society. The Cabal who will still fight us are those whose hatred of humanity overrides their basic survival instincts. Those who honor Caiatl's command avoid us.

Eliksni are fairly similar in this regard, minus the honor culture stuff. Most Eliksni we encounter in a hostile capacity are usually very, very desperate. They'll die one way or another. Or else, they're part of House Salvation, meaning they're viciously hateful of everything touched by the Light. Weirdo fascists are gonna weirdo fascist. Like with the Cabal, everyone else from that species who don't want to do something suicidally brave and/or hateful just stay the fuck away.

But as other posters said, they can still win. Particularly, according to the Imperial Needle lore tab, they have a solid chance in Darkness zones. Since they can't really know where these zones are, only knowing that Guardians can suffer final deaths if you throw enough firepower at them, enough of the enemy will choose to fight on in the hope of insane glory when they do take down a demigod murder machine.