I don't think anyone has heard it all. People with an interest in human nature, especially in positions involving management and otherwise dealing with many people, get to hear and recognize main trends, sources of suffering and emotional blocks, personality etc, and can develop a level of immunity to be personally overwhelmed and traumatized vicariously. Or the opposite: become more open and receptive to others' sharing and experiences. But the number of variables are virtually endless and every interpersonal experience is both unique and novel in many ways. I think if someone says they have heard and experienced it all can be more a sign of burnout and apathy rather than realistic perspective. It is very possible to arrive at a level of perception and life experience where things do not shock and overly disturb us though. Be it a therapist or not. I personally feel that in my own life, the most effective "immunity" of this kind is coming from experiencing my own challenges, limitations, periods of being stuck, despair, and aging itself. Naturally, I tend to relate more deeply and realistically to others who describe and have been through experiences similar to my own, but primarily cognitive understanding and knowledge acquired through reading, listening, and navigating a broad variety of interpersonal relationships also helps. I don't think it's unique to therapists at all and there are Ts who are just unable to hear and deal with certain things - I had one of those.