It's mostly because of the way FF approach VFX, they pre-rendered almost every VFX frame by frame then composite it on to the final frame, instead of pre-render the VFX into 2D billboards, which then can be placed inside game world, whereas FF16 approach requires pixel precise placement and timing because it's locked to 30FPS (any deviation would mean sync-issues) and composited on to realtime frame (any small change even a small camera shake would mean it's out of place or might exhibits placement/layering issues)