It's actually better for a melee to focus one target. We have to worry about positioning for our rotations and combos, having to then cycle through othet targets and move is just a hassle and lowers DPS. Also, focusing on a single target is more TP efficient.

People can come in with their fancy stats, etc, but this game isnt about flat damage.

Also, the quicker a mob dies, the less damage it does, the easier the healers job is, dont be an inconsiderate tank. You have just as equal, if not more important role in managing your health.