Because you don't have to be a programmer to be able to tell us what makes them a fundamentally different character.
You're ignoring my point. It's not just their hitboxes, it's what their hitboxes
and everything else that makes their gameplay possible are working towards. The final result. It's the same reason the
Fighting Polygon Team exists and operate differently from one another. Despite looking very similar to one another, you can immediately tell them apart from each other. That's because what they look like doesn't matter, it's their movesets that do. Hell, if Smash Bros. never existed and all we had was a fighting game with the Fighting Polygons, we would still be able to tell differences between the polygons. And I'm not talking about the subtle differences in their models, I'm talking about how the way their movesets work makes them feel different from one another. Just as how you can look at data and tell it apart, so too could you look at generic, seemingly random polygons and distinguish them. This isn't an impossible concept. You
can tell me how Kirby and Bowser are different characters beyond what they
look like, right?
I'm not saying it's impossible to bring something new with a katana. Again, appearance does not limit creativity. I never once said anything of the sort. What I'm asking is what Takamaru would do to feel different. It's not enough to have a distinct weapon, otherwise you're telling me that a character can be a clone of Dedede and still be
undeniably unique simply because they have an axe. Their weapon of choice doesn't matter, what does is how their weapon affects the gameplay.
Dedede, a slow, heavyweight character that makes use of powerful, large-hitboxes and excellent recovery. Look, I just explained what makes Dedede unique in one sentence without needing to mention anything about his hammer or his appearance. Dedede's hammer could have actually been one really fat arm for all we know from that description without knowing what Dedede looks like, yet we still have an idea of what his gameplay would be like. Here's another:
Meta Knight, a lightweight character excellent at aerial combos comprised of rapid smaller-hits to build up damage. Again, not a single mention of his weapon or his wings, nor does any other character. Using the same broad terminology, describe Takamaru the same way. If you struggle to name a concept another character doesn't already have in Smash Bros., that should say something about how Takamaru's uniqueness is only skin-deep.