Because it’s not Odas style. Dismemberment is designed to be a gruesome thing which is why it’s only really done by villains (doffy, Kaido) or an act of self sacrifice (Shanks, Zeff)
Having it done or even more so completely cleaving through enemies to kill/critically wound them lowers the impact of when it’s done.
Above all, One Piece isn’t a seinen, it’s not going to involve those gruesome scenes. But you can ask the same thing about swordsman of other shonen manga as well.