I don’t know whether my commenting this will really affect anything, but these are simply because of how things work and are built in the game.
The players’ attacks are modelled somewhat differently from the enemies’. We have explicit cleave values, and some things (like hitting a shield or heavy armor) can stop that cleave prematurely, so you need some tactical thinking in how to deal with mixed hordes. I don’t think there’s any attack from the player characters that is strictly single-target; even the ones that are practically so can cleave a bit, they just have little chance of doing it effectively because of damage limitations (with one-handed axes and daggers) or attack paths (with stabbing attacks). There also are some genuine AoE attacks that can and will hit anything in the area regardless of their defenses.
The enemies’ attacks are modelled more simply. They’re either single-target (anything from the trash, berserkers or most Specials) or AoE (most Boss attacks, some Specials and all or most of the other Elites’ attacks. The AoE attacks are something that everyone needs to block for themselves, and even if the area is small (about the size of the weapon’s business end), the Elites’ overheads work as such. There are no distinct cleave values for enemy attacks; they will either hit only one character or everyone in the area.
This leads to another tactical consideration: Primarily, everyone needs to keep themselves alive. so check your surroundings, don’t trust too much on your allies keeping things clear, and only when you’re in the clear (even momentarily) go help them specifically. as j_sat and others have put it, keep your head on a swivel.
Yes, I know it’s annoying and somewhat unintuitive (and allowing shields to somewhat protect others could indeed raise their popularity), but any changes would add some more (computational) complexity to the game and thus, that’s how things are for now.