Thought that just came to me: If Roxas can use the keyblade because he is the nobody of Sora, then why can't Xemnas use the keyblade? Both Terra and Xehanort are keyblade wielders so shouldn't he be able to?
Well as above me said, Terranort lost his memories and with it the keyblade(s), (also teased Eraqus' heart is within Terra's too) and while it was teased Xemnas did remember some of the past, it didn't seem he remembered how to summon the keyblade at least.
Only thing he did manage to "summon" at least was Xehanorts armour.
What I think is more confusing to think about is why didn't Xehanorts heartless/nobody look like any of the people within his heart?
Like Sora's nobody Roxas looks like Ven, who at least he shares a heart with (?), while Xehanorts heartless/nobody look like a mix between Xehanort and Terra.