A
Azrael
Guest
This I agree with you :\ but aside from the point of him having no one in his Heart - the captured Princesses included - you do have to ask yourself how the Floors were event crafted, and if you tell yourself that DiZ just reused the cards they made for Sora (because he couldn't really create new ones for Riku), while somewhat lame, plot wise it makes sense.
It's a crappy excuse nontheless and it would had been nice to visit the worlds Riku went through that Sora didn't or a world crafted from the Realm of Darkness.