There could be a special reason that you might hit on downed PC(depending on initiative order), but in addition to trying to eliminate still active target, you also force downed PC companions to spend non optimal actions.
I.E. it is better to force the paladin to use cure wounds(any level) than killing paladins comrade and than suck on 2 attacks from a greatsword with 2 smites added on.
Now, AoE attack are great for downed opposition, you deal "death save fail" in addition to dealing HP damage to still active opponents.
I feel that, regardless, that's 1 full-action less than what the monster would have been hit by. And multiattack picks up the slack when that 1d4+5 HP only takes 1/3rd of the monster's action to reduce to 0 again.I think your analysis is missing an aspect. Sure, as a monster, it's better to force the paladin to heal than have it hit you in the face. But that comrade who gets healed by the paladin... THEY might hit you in the face (or cast fireball or...). This is why bringing back a downed ally can be a good move from the Paladin's perspective.
That's a great point. How exactly are the PC's determining the NPC's are dead when they drop to zero hp. Why aren't the PC's wasting attacks on them trying to make sure they are dead? It's because we all inherently know that PC's can tell that NPC's are dead. For all the real world talk about how we cannot tell when someone is dead, it seems a bit odd to have PC's that can do so and to have no concern for that.If PCs can tell fallen PCs aren't dead, then NPCs can tell fallen PCs aren't dead.
I know whenever I make death saves for a fallen NPC, the PCs go all out to finish them off. Having NPCs not target fallen PCs is the GM saying "I don't want to hurt you".
I wish we had official options to support that or even death at 0.I'm talking with my players about switching to -10 hit points for the next 5e game and just getting rid of stopping at 0 and death saves. A lot of the low level healing whack-a-mole spells might not bring up a PC who is at -7, and further, even if you get 9 hit points back, do you really want to just hop back into combat with 2 hit points and death at -10? It will also help me make the game more dangerous without feeling like a bad guy for marking a downed PC for death.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.