I do agree that Wilt, Ed and Coco would each make stellar imaginary companions, but I'm sure that an awful lot of us would miss them if they disappeared from Foster's (and adoption would mean removing them from the show's central setting, which would certainly shake things up a bit). I'd say that it's potential subject matter for the final episode, but until then, I don't know, I have this hunch that all of our favourite friends are going to be sticking around where we can see them.
EDIT: Note that none of the recurring background characters - Jackie Khones, Fluffer Nutter, Bloppy Pants, etc, ever seem to get adopted either. While there may well be individual reasons, within the context of the show, why potential adopters should find them less appealling than others, all in all I'd say that it's simply more convenient, for the purposes of character and continuity, for them to remain.