You know, that has me thinking.
As revealed in 358/2 Days, Roxas, when first born as a Nobody, was like a baby. He didn't know who he was or what to do; as such, he was very impressionable. So impressionable, in fact, that he was easily recruited into Organization XIII with little effort.
Now, what I'm thinking is, all Nobodies when first born, especially those who retain human form, are just like Roxas was. So it would be very little effort on Xehanort's part to convince them of what happened. In fact, even if they retained their memories, he could brush it off. And, maybe, they don't remember the instances of their demise that led them to a "life" as a Nobody. So that could easily be a part of it.