Why would CEV operate on humans that do exist, and not on humans that could exist?

To do the latter, you would need a definition of "human" that can not just distinguish existing humans from existing non-humans, but also pick out all human minds from the space of all possible minds. I don't see how to specify this definition. (Is this problem not obvious to everyone else?)

For example, we might specify a prototypical human mind, and say that "human" is any mind which is less than a certain distance from the prototypical mind in design space. But then the CEV of this "humankind" is entirely dependent on the prototype that we pick. If the FAI designers are allowed to just pick any prototype they want, they can make the CEV of "humanity" come out however they wish, so they might as well have the FAI use the CEV of themselves. If they pick the prototype by taking the average of all existing humans, then that allows the same attack described in my post.