Its length would also be an issue, I think. Above all, it's the ease of having to slit someone's throat with a very long blade--whether it be rapier or katana or spear. Excuse me, let me show you--

[ She flies up to him, holding out her bare hand near Hinata's throat ]

Spears are designed for throwing and thrusting, so the movement would be awkward if you used it like a knife.

[ She shapes her fingers into a point, and mimes...the effect of trying to use a spear like a knife. Hovering a little over his throat area. ]

Katana-like blades are easier, but also difficult, depending on their length. Wakizashi and tantou are closer. [ Miming dragging a broader blade, again ]