So why can't we control more than one character, if the possibility exists of playing as more than one character during the course of the story?

This single player FF is gonna be a strange thing for sure, but I'm not ready to discount the game just for being limited. Art thrives on limitations. If you don't have to input commands for the other party members to attack, the game becomes a single player session of Secret of Mana.

And that's ahem, nothing to sneeze at.