Monk is actually the highest dmg in the game on boss encounters. The problem is that they require much more effort than everyone else to put out that dmg.

There is no breathing room for error. Monks can't afford to make many mistakes or flub their rotation and lose greased lightning stacks or their dps will plummet. Other classes don't have the positional requirements, melee range, and complexity of the monks rotations/buffs to keep track of so it's much easier for them to produce numbers.