Ryokugyu ain't bringing back someone's blade and for him to have one without being able to use its benefits is nonsensical.
There are only 3 World's Strongest titles, is that also nonsensical?
Only two characters were strong enough to create Black Blades.
Zoro's ancestor and character he is based on and Zoro's target.
The only other person capable of doing the same is Zoro himself.
They are that rare because they are marks of ultimate Power, Sky-chin. Yoru isn't #1 weapon in the world for lulz.