It might not be the reason why, but perhaps they had the same system in mind that they used in ffxi, where you had job traits like fast cast and job abilities that lowered weapon delay (and maybe in ffxiv's case, tp attacks' gcd), and then haste that would increase the speed both of those aspects at the same time. Maybe in the future we might get these types of things too, and a haste spell, that might make the separation make more sense.