Because I wanna enjoy the game when I am playing it with me mates instead of the gameplay being a snoozefest.
I think "more buttons" is the only half-decent way FFXIV has of going at it from the rotation point of view. I consider mashing buttons bleh, and you can't really make the execution too time-sensitive due to how shit the back-end is.
They could make it more reactive based on what is actually going on with the fight (i.e. look at what's happening on the screen instead of what is shining on your hotbar), but that might be beyond how the game was built.