Celebrim
Legend
If you watch any Superhero or fantasy movie nowadays, there's a consistent trend. In most fights, the second the combat starts....the heroes go their own ways. Legolas isn't back to back with Aragon and Gimli, they are off killing their own monsters. When the Justice League (both in movies and the cartoons) goes to take on the badguys, most of the time the heroes all split up into 1 on 1 type fights. Only when they are facing the "big boss" they all start attacking the same creature as a single unit. If we go more modern, Harry Potter often had the wizards split up into 2 on 2s or 1 on 1s, rather than have 1 pile of wizards go after the other.
The appropriate response to this is that movies are terrible at displaying realistic combat. Movie combat is based entirely on narrative concerns and is often heavily divorced from reality such that if you analyze it, there are usually a lot of ways one side or both could have more easily won the fight. For example, in fights between a hero and a group, the group almost always stands back and takes turns fighting the hero. They don't really team up and attack together. Realistic fight combats rarely happen.
That said, part of this a feature of D&D in that D&D unlike some other systems doesn't make you pay any price for ignoring a potential attacker. In say D6 Star Wars, if an NPC has reason to believe they won't get attacked, they can decide not to spend a dice on defenses and therefore become a more effective attacker. Covering fire to force NPCs to reserve defensive actions is important in a way that it isn't in D&D.
In my 3.X game, I deal with this by having fighting stances - offensive, balanced, and defensive. If an NPC feels like they aren't going to be attacked, they can adopt an offensive fighting stance and trade AC for an attack bonus. This can be used to punish PC's for focusing entirely on the obvious target, which of course can also adopt a defensive fighting stance to resist the PC's attacks. It doesn't negate focusing fire as a strategy, but does make the decision more complicated (at the cost of making the game more complicated since PC's can declare stances as well).
That being said, lots of things that occur in fiction are difficult to model in RPGs because PCs aren't protected by power of plot and rule of cool at all times, so the PCs generally have to adopt more "boring" realistic strategies most of the time. You could give the PCs plot protection and access to rule of cool at all times, but that turns to not work out either without a lot of work as things that are cool once are less cool the second time.