An interesting hypothesis is that the effectiveness of therapy has an inverse relationship with the culture’s acceptance of therapy as a practice. In other words, if the culture is intolerant of therapy, and sees those who go to therapy as "wrong" in some way, then the potential for a place where one can be accepted—instead of judged—and say anything that comes to mind will most likely be of enormous, almost inconceivable value.
On the other hand, if we lived in a culture where the expression of one’s vulnerability was a common thing, then there would be relatively less need for therapy. Relatively because a professional might still handle these vulnerabilities more effectively, for the obvious reasons of training and experience, but also because of logistical reasons, e.g. being unconnected to the person’s life.