How can they add more classes and make them unique and have their own flavor?

[dinnertime]

Yeah, I mean I know they have a few differences, but they are ultimately the same in my eyes. Like NIN to MNK to DRG is more similar than WAR to PLD, SCH to WHM or BLM to SMN - at least from my perspective.

Yeah, I can see how they seem similar to you since the other pairs you've mentioned have entirely different gameplay when compared, but pretty much the differences between the melee DPS are what kind of damage they do (NIN is slashing, MNK is blunt and DRG is piercing) and the ones I've pointed out. That at least makes them pretty different to my eyes though.