I think the "therapy is not for everyone" notion is often used as a convenient rationalization for negative outcomes. It's a way to blame the client without blaming the client. And it's usually packaged with suggestions about the client not working hard enough, or trusting enough, or believing.

I'm with the OP -- training in psych and behavioral theory does not automatically translate to anything of value clinically or to any sort of ability to help another human being. And yet the profession depends on this conflation to sell it's product.

Like Misbella, it was only after diving in to some of the literature and online content that I could see any of this.