Because there's nothing skilful about that - the timing window is extremely long, and it just requires memorisation of specific skill combinations. You cannot really adapt to it as interactions do not proceed predictably. Mob weaknesses again are simply knowledge.

DPS in this game have loose rotations with a much tighter timing than FFXI (granted, it's still very easy compared to games like Rift or WoW). You are forced to vary your approach based on number of enemies, disconnects, mechanics, etc - this requiring you to change your ability order as you go.

In FFXI, multiple mobs (unless you cleaving, a BLM or a BLU) made no difference to anyone but someone trying to CC them, and every fight is approached identically. Engage, afk to 100 tp, WS, repeat. Throw in skillchains if you want but they don't actually increase your damage in most cases. Unless the two melee are equally geared etc, you are just weakening the stronger melee for more than the skillchain gains you. BLM is also far weaker than another melee so trying to capitalise on skillchains is counterproductive.