Simply risk and reward, a monk losing GL will cause him to lose a lot of dps where a DRG who lost HT can simply reapply it asap, same with NIN, they have their mudras, if a Huton mudra fail that's 20 seconds where they will deal a lot less DPS. Reaching top DPS on a MNK and NIN is simply more skill compared to DRG which is why DRG is lower than both of them, though I agree that DRG need one more kind of raid utility then they'll be sitting at the same spot as the other two DPS, not that there's a huge difference though, the melee DPS are all quite balanced atm.