(near-entirely theorycrafting/hcs; only one sentence in this entire rabble is canon)
Keyblades are, at some level, alive and/or sentient. They act on their own, often dragging their wielder along with them. When harnessed properly, this lends itself well to aerial combat and long combos.
Kirby doesn't shy away from creating life, as seen with some of his Helpers, but he avoids making a copy of someone. He figures making a copy of someone might make them sad when they find out they're a copy, or make the original uncomfortable when they find out. As such, when he creates Helpers, he creates a randomized iteration of the species based on examples that he's interacted with.
Sora's Kingdom Key is unique. For a while, it was the only Keyblade Kirby's interacted with. Even with Game & Watch dragging one around sometimes, he doesn't have enough to go off of to feel comfortable replicating it.
Instead, he makes little sparkles in the shape of one in battle for the showmanship. Everyone loves sparkles, after all.
Oh I looove these ideas they all make sense
