Funny they don't mind that when it comes to Cloud's outfit. Or Squall's. Or Zidane. Or Yuna/Tidus if I"m not mistaken. Or Noctis...or Lightning...
IMO it's probably just to save development time from having to make the clothing dye compatible and they use that as an excuse. Which is understandable I'm not demanding they make them all dyable, but I'm not sure I buy their reasoning given the above.