Can't speak for all the others you target, but as for Gerson, they have plenty of studies that back up their method. Since Gerson's studies manifest in actual live humans, by the thousands, still living long after the fact that that orthies gave up on them and their disease, and does not dwell in the static scientific studies that orthies are comfortable with, orthies will probably view this kind of study as foreign and strange to their presently molded understanding. In other words, Gerson has enough of their own actual live human case studies in progress that they are plenty busy enough as is without having the luxury to spend time getting caught up in sponsoring a scientific study.