You may know that you need to move at a specific time, but you won't always know exactly where to move. Maybe the boss places a delayed AoE on a random player, or it fires a wide beam in a random direction. You know the attack is coming, but you still have to watch the boss to know where it's coming and react accordingly. Some examples:
- Sophia ex's duplicated attack mechanic - it's not scripted which clone you have to move under, or which attack she does (although being close and just behind avoids both)
- Midgardsormr's spin/roll attacks in O10. You have to watch what he does to know whether you need to move close, far, to sides or to corners.
- Midgardsormr's flame breath mechanic just after the add phase. The order in which players are targeted is random.
- Omega's starboard/larboard mechanic in O11. The boss flipping around between the two attacks makes it even more difficult to get straight.
- Several boss mechanics in the Copied Factory. All bosses have mechanics where the targeting of an attack is randomized and you can't just move to a fixed place at a fixed time.
And let's not forget the perhaps most important source of randomness: other players. You can never predict exactly what they do, and it has a significant effect on where the attacks are aimed. Only raiding with a static alleviates this to an extent since you get to know each other and can do more planning beforehand, but there are still factors like someone being drunk or tired which may make them forget or react sluggishly and then others will have to adapt.
Also, even if many of FFXIV's fights are loosely choreographed dances, they're incredibly complex dances. Some people (me included) get satisfaction from mastering such a dance. It's kind of like playing an instrument (piano in my case) - a particular song is always the same, but it still feels good when you can move your fingers in precisely the right way for several minutes without making a mistake. If that's not your thing, that's fine, but it doesn't make FFXIV an objectively worse game.