I don't think it would be very accurate to say every other weapon is made the same and the katana is the only one that isn't.
All the weapons follow the same method of construction but have you noticed the animation for it etc is the same? It is assumed that, RPwise, your character knows how to make a katana the way it should be made; same as he knows that making a longsword is not done the same way as making a maul, or a dwarven waraxe, or even a sai for that matter. You simply have to roll with your character either knowing how but following the laid out mechanics or your character not knowing how and so don't craft it until you feel you've earned the knowledge (if you are so inclined). Changing it for one would mean having to change it for others and ultimately, the novelty will wear off and its probably not worth the effort then. It is not as if changing what is needed to craft one or how high you need to be in weapon crafting will really stop anyone or make crafting it all that more interesting after one or two goes. The fundamentals will remain, go here, farm that, return here, roll dice. Unless you change the whole system JUST for one weapon in which case it will take a lot of time for very little return on the investment.. not to mention others who may like other weapons and read more into them will request the same. Besides, NWN was never designed to have an intricate and complex crafting system, it follows a formula like most of todays games.
