Casters can do some huge numbers and the trade-off for that is that their gameplay requires them to stand still for most of that time. This is true of every caster in every MMO.

Especially for BLM, the thing that makes the class interesting and hard to play is knowing when you can turret and when to save your Swiftcast/Triplecast.

The opposite side of the coin is Physical Ranged. They have lower overall DPS BECAUSE they are so mobile and don't have positionals. If their DPS were on-par with the other DPS they'd be absolutely broken, as would be casters if they weren't constrained by their cast times.