The true reason is that Square Enix is secretly making a diving simulation game and they planned to use some of the basic mechanics in FFX before realising some people mightn't spring for their future diving simulation game if they can get a simplified experience free in another game.

Realising some gamers were so lazy that games that are pretty much interactive movies would sell well led to the development of the system used in FFXII. It all makes so much sense!!!