Yeah, it's because Yoshitaka Amano designed the characters for FFI-VI and IX, while FFVII, VIII and X were designed by Tetsuya Nomura. Tetsuya Nomura was also the character designer for KH. Because of this, Amano's characters will probably unfortunately never be used in a KH game. This saddens me greatly because I think it'd be awesome to have some of the other FF characters in the games, like maybe having Cyan or Yang on your team along with Beast or Mulan or someone equally cool.