Because people are picky and like to whine about insignificant things.

I enjoyed all of the characters' voices except Tidus's, although after playing through the game you kinda get used to it. At least Tidus puts emotion into his voice, unlike Vaan from FFXII, who sounds like he has a cold all the time.

Admittedly, I was too busy reading the subtitles to worry about the mismatched lip syncing thing.