Why you ask?

That would be because everything that is super awesome about SCH has to be cancelled out by something super lame to keep SCH from returning to God status. If you've ever read up on Napolean Bonaparte, you will notice some similarities between these two military strategists. To over simplify it, they are basically a double-edged sword.

What I mean by that is something like the constant auto regen she brings is negated a bit by having to cast her, and she also costs MP to do so, and triggers the GCD when you do; but I digress her Auto regen is really good, but not so good to absorb the impact of having to cast her mid battle.

Having to cast Fairy Summon mid-fight is more of a penalty. You know you messed up somewhere if you have to do this. It used to be worse. Skills split between fairies forced the SCH to have to switch fairies if the wrong one was out. They had a longer cast time 4.36s iirc. They could be KO'd making you have to resummon. And they would have to be resummoned by you after Dissipation effect ended.

They fixed all that, and there are still issues with the Fairy lol