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

[Alahra]

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.

Since you don't like (and I assume don't play) melee, the differences probably aren't as apparent "from the outside."

In addition to their damage types and their position on the DPS "totem pole," each class's combo systems are different, and their priority chains are of vastly different lengths (MNK's is the shortest, NIN's is in the middle, and DRG's is the longest). Each one has a varying level of utility, as well (with NIN being the highest, with proportionately lower personal DPS to accommodate that, and MNK having the least, and the highest personal DPS). Each one handles its maintenance buff (GL, Huton, BotD) differently as well, with DRG even have random/proc elements in their rotations, which NIN and MNK don't have. They also emphasize different approaches to DPS—MNK is all about long periods of sustained DPS growth, DRG is about punctuated burst, and NIN is more about increasing raid DPS for specific moments. The way they generally interact with positionals is also fairly different, with MNK requiring far more movement than either NIN or DRG, and NIN requiring less than DRG.

They play fairly differently in practice. That makes it harder to transfer the muscle memory from one melee to another, which is part of why you don't see a lot of players that play more than one melee seriously, but players who can play multiple healers or multiple tanks well, for instance—using whichever is better for a given encounter—are more common. Of course, that the roles share gear for the others helps, but if you couldn't transfer your basic knowledge from one to the other, it wouldn't be as easy to switch for the others, either—after all, DRG and MNK share accessories, which reduces the barrier for entry, but the classes are distinct enough that players who play both of them equally are fairly rare.

That isn't to say that, for instance, WHM and SCH are overly similar, either, only that they share a core set of abilities in common (with Cure/Physick pairing, Cure II and Adloquium pairing, and so on). The melee don't really have much that's like that, with the only real overlap being a gap closer and a stun, but the differences between their comparable abilities here are generally greater than those between the shared abilities that the tanks and healers have.