Hmm good question. I can understand them not changing in the field, like in the games where they tent for the night. FF8, they changed clothes some in the beginning. But what about when they are on airships or actually home for a change? For example, Aerith went home in FF7, why didn't she change? Lol wouldn't that be intersting, everytime you tented or slept for the night random team members would change clothes. Then, if you didn't like what someone was wearing you could just tent again and change their clothes lol.