Myths, like some religious traditions, limricks, maxims, etc, are often perpetuated to get people to do the right thing, even if they don't know why they are doing it. A form of social control for the deemed benefit of the often uneducated masses. In this case, almost none of the aforementioned techniques described in the myths (except for the potato deal) are fundamentally wrong. They each have their own usefullness, but are poorly explained by the myth. It is 'easier' to explain that searing meat seals in the juices, rather than to explain the caramelization process, the need for avoiding over-manipulation and poking, etc. Though the meat is not 'sealed' per se, one cannot deny that searing does produce a juicier cut of meat. By telling people not to wash their mushrooms, people won't be tempted to let their mushroom soak in a water bath for hours at a time, which I garantee will significantly increase their water content, even though a gentle rub under running water won't affect them one bit. In some contexts (where knowledge of science is absent, for example), this is the only way to ensure that sound culinary (or other) principles survive the generations.
