You have to look at the entire kit.

WAR, for example, has:
Berserk for 50% extra damage.
Maim for 20% extra damage.
Storms Eye for 10% slash resist down
Abandon Stacks (up to 5) for 2% Crit each stack.
Can cross class Internal Release for more Crit.
oGCD of Brutal Swing.

Look at your PLD example. While yes, Goring Blade has high total potency, odds are if you're doing the Goring, RA x2 combo set, you're clipping the DoT.

You also only have Fight or Flight, the Sword Oath auto attack bonus, and Spirits Within for extra damage potential (off tanking wouldn't proc Shield Swipe so I'm not counting it).

DRK has several oGCDs, constant 20% boost from Darkside, and potent DOTs.

You can't say "since PLD has the highest total potency attack, they do the most damage".

If this were true, then they would have more damage than DPS classes, but they don't.