I am pretty sure a large number of them are, or are becoming so, but they have boxed themselves in and do not see a way out.
They, and others, have allowed the entire birth certificate, natural born citizenship issue to be defined by the media as a whacko conspiracy cult delusion, and are unwilling to speak out lest they be painted with the “birther” label.
I expect they would be delighted if some form of explosive proof were to discovered and publicized sufficiently to allow them cover to speak out, but until that happens, they are not willing to touch this issue.