That they are all too vapid to be wrong!
Freud and Piaget did not test any of their " theories " but just converted their observations into dogma. Dogma that for the most part is long refuted. Defensive mechanisms are what is left of Freud in the modern world.
Mead spent six months in Samoa, was fed a ridiculous story by young girls about their sex lives and wrote a nonsensical book that was an insult to Samoans and refuted by Derek Freeman.