Why are monks consistently lowest damage in CC?

[Gserpent]

MNK isn't bad at all. It posts lower damage numbers because it has no poke and virtually zero incidental AoE damage.
MNK is a competent single-target brawler at all times, and a deadly assassin when its ult is up.
Total damage isn't a particularly meaningful stat, especially if you're comparing one job to another instead of comparing, say, two MNKs in the same match.
SCH can easily deal double or triple a MNK's damage over the course of a match, but it's irrelevant because it's all DoT padding.
By the same token, BLM and NIN can get their best total damage numbers by playing the jobs completely wrong.

The AOE DoT isn't really padding, though. Even unbuffed it's essentially sapping a Recuperate from everyone it hits, plus the damage down. Buffed up, you're looking at 25k damage, and up into the 30k range if there's a BRD or AST around. SCH is an attrition/pressure class. They focus on fudging numbers in their team's favor with constant overshielding and damage buffs and debuffing the enemy team. SCH rarely gets kills but their pressure output is intense.