Theatrics, theatrics, theatrics. Final Fantasy VI is an opera. For the party to just walk into Kefka's inner sanctum and rush him wouldn't have been dramatic enough. The evaluation of the scene shouldn't be how good their answers were (philosophically speaking), but how much feeling is in the answers, what imagery they evoke.

In other words, it's possible (as BN pointed out) that the more educated among the party could have responded with a philosophical counterpoint to Kefka's assertion that life is meaningless because nothing lives forever...

...But where's the fun in that? It's so much more sweeping, so much more epic, to respond to a coldly calculated view, one that says nothing is worth living for, we're all cattle waiting to be sliced by the scythe of death, to respond to such a view with feeling.

Because that truly is what Kefka, as a character, lacks. He is completely out of touch with anything resembling depth of feeling. He thinks life is meaningless because he's lost the ability to quantify it in terms of how he feels towards it, due to his lack of empathy. That's the real truth that he isn't stating, because he probably doesn't himself realize that's why he's come to the conclusion.

It's this "missing link" in Kefka's chain of reasoning that is the root of his madness. The cast, in responding to him, are providing the moment of drama, of crescendo, by illustrating to the player just how far Kefka has fallen, what he has lost sight of.

I.e., why it's necessary to beat the living turds out of him until he is stone dead.