Because then you'd spend more time refreshing your dots, and less time using a filler like ruin to increase your dps. Or, if movement is required on the fight, the more dots that you have rolling at once, the higher your damage will be, even if you are unable to cast while avoiding AOE attacks on a fight like Titan or Ifrit.

Hypothetical scenario, you can only cast 1 spell, and then you have to spend an equal amount of time moving.

If you do your rotation, you will not be able to cast Bio 2 or Fester until 30 seconds into the fight, because Thunder will have already faded before you even got to Bio 2, and then you'd have to reapply Bio 1, because it'd be fading at the same time... So at this point, 44 seconds have elapsed before you cast your "first" fester...