Well back when I mained ninja I noticed that when I place doton on the ground, the tanks immediately move every mob out of it. Additionally some classes just have very low AoE dps. In some cases it is probably faster to single target each mob. Like with red mage for example.

Also nobody is pointing a gun at your head and forcing you to do big pulls all the time. It's not very fun to see the tank kite a bunch of mobs over half a dungeons that you can't attack on the move while you see your huton or dualcast evaporate.