Do you even know how a "default" Keychain works? Where did the Keychain come from?
We can assume where Riku's WtD Keychain came from. The amulet on it being the x-less Heartless emblem and it being the Soul Eater with angel wings slapped on it is a dead give away.
We can rather easily conclude where Mickey's original Star Seeker came from, seeing how Yen Sid was his apprentice.
What about the KK and IKK, however? Notice how their designs are also identical to a degree. How Riku was chosen by Terra for that Keyblade - in a way, had it given to him until he could use it. How Mickey had to go and find that Keyblade - which means he knew of its existence before becoming its wielder, one can claim.
We've seen wielders gain their abilities in a natural manner, and then their Keychains materialized out of their own Memories.
However, the KK and IKK bring about a possibility one cannot ignore, which is that Keychains, once formed, can take on an independent presence. It would certainly tie for better or worse with how people can "give" Keychains to Sora...