This has a lot to do with the set up and how the DM makes and runs encounters. If you make an easy target for the players to focus fire on....then that is what they will do.
In action movies the group of heroes splits up to fight the group of villains that oppose them. OR the heroes split up to each take on enemies (tailor) made for them. An perfect example: The Avengers movie. Thor and Hulk go after the space worm dragon things, Iron Man zooms around with the battle skiffs, Hawkeye takes out targets of opputrunity at a distance, and Cap and Black Widow are on the ground, hand to hand fighting. You might notice the clever writer gave each character enemies made just for them.....
When making an encounter, it's easy to make two or more main foes. Even with two foes, the players would be a bit foolish to attack only one and leave themselves open to the other. Also, you might want to add foes of different types...made for each character. Large masses of foes work too.
Lots of monsters have abilities that can be useful. A flock of preytons, for example, are silly to ignore to 'focus fire' on the naga.