Thinking like a monster
There is no doubt: focusing the fire is a good idea. No matter how you see it, math is in your favor if you try to hurt an enemy until it is shot down, and then move on to another. Once you realize this, it may seem that you have to use this tactic or be completely unreal to what an enemy would do. But there are some reasons why a group of enemies could not use the focus fire. And not all this is "fuck the rational: I want my PCs to survive." Sometimes it can be more realistic for your monsters to avoid the focus of fire. Here are some possible reasons why they could do it.
1. They are used to weaker enemies.
Many "evil" creatures take advantage of creatures much weaker than they are preferably. A squad of goblins could be used to attack only the sick, the weak or the unarmed and unarmed: merchant families or innocent peasants. More important, they may be accustomed to enemies that fall at once. In a situation like that, it is actually disadvantageous that the goblins aim at the same enemy: their first save could end up hitting an enemy with five arrows when one had killed him, and leaving other targets unharmed. By extending their attacks, they ensure that they eliminate as many enemies as possible in the first round, ensuring that no one escapes and leads to a long and disorderly hunt. Perhaps these monsters use these tactics by default, only realizing in part of the combat that these opponents do not fall so easily.
2. Enemies act at the same time, but don't think like one
Trained adventurers can act in groups as a perfect unit: adapt their tactics to those of their allies almost instantly. But minor enemies (again, for example, goblins) fight without discipline. Perhaps all goblins think they should focus the fire: focus on one enemy. But each elf could have a different opinion about what enemy it should be. Three of them could shout conflicting orders at the same time ("Shoot the human with the bow!" "Shoot the bright dwarf!" "Shoot the elf who set me on fire!"), And different elves could follow different orders . In any case, the fact that all these enemies act on the same initiative could justify their confusion about what the other elves are doing: They act almost simultaneously and do not have time to notice the actions of their peers or to coordinate their efforts.
3. Monsters are selfish and silly.
Naturally, this depends on the particular enemies you fight with. Some enemies may be inclined to focus the fire despite low Intelligence scores due to their natural instincts to focus on a weakened enemy at a time (such as wolves). But in general, some monsters are not that smart. Even some very intelligent enemies (such as vampires) may not be used to fighting in a group, and may simply fight using their own priorities, instead of trying to use teamwork. Some monsters fight more using anger and thirst for blood than tactics and intelligence: even if those monsters are intelligent enough to plan and think tactically, they can abandon these plans in the heat of battle and attack only creatures that hurt them or those that seem smaller, or those that seem to have the most valuable equipment. The three monsters that were hit by a fireball could point to the wizard, but the four that were not could focus on the Paladin to plunder his shining armor. Bottom line, The motivations of the monsters are not always to make the movement more intelligent: sometimes it is to make a movement that satisfies some lower instinct.
4. Monsters don't know what hit points are
This is a large one (and is a kind of variation in point # 1). "Rationally," every arrow that fires a monster is an arrow that they hope to find the heart of their enemy, and leave them dead, regardless of whether it is the first arrow fired at an enemy or the 50. Monsters don't know they are in a game , and that it is statistically impossible for the first arrow of an elf to throw a furious barbarian of level 5. Monsters can extend their attacks because they expect each attack to kill an enemy. They don't know that "1 hit point" is a state that an enemy can reach: they only know that they hurt their opponents until they fall.
Similarly, we (as DM) know what the best tactics are against this particular group, but the monsters may not know. For example, enemies may not think of staying scattered enough so that only two of them are within a radius of Fireball at a time: why would they do so if they had never seen these enemies (or maybe others)? use fireball? Similarly, it may not be particularly obvious to monsters which creatures have a lower maximum HP and which do not (keep in mind that HP often tracks resistance and drives as much as physical damage or endurance). So, They can make decisions that you, as DM who knows everyone's statistics and skills, know is a bad idea: but it is not unrealistic or irrational that they behave this way, because They do not have access to your information.
But sometimes … focusing the fire is the way to go
Keep in mind that all of these reasons (other than # 4) are conditional. There may be monsters that are intelligent and / or great when using teamwork: they can point to the weakest enemy as a pack of wolves that eliminate the weakest member of a pack. Focus fire can often be exactly the kind of tactics your monsters should use, in which case, go ahead! This may indicate that these enemies are particularly well trained, disciplined or simply dangerous.
When the correct tactic is empirically obvious to us as DM, it can be difficult to make our intelligent creatures behave in a tactically unsound way, while we strive to make their thinking realistic. But it is worth remembering that realistic thinking can be exactly the kind of things that would lead to bad tactics. After all, people (and monsters) make bad decisions all the time.