They do not achieve it every day with every client. Also, it is a different experience for them than for the client. Finally they are trained on how to it, at least one hopes they are and they do hold themselves out as trained in something.
I don't think it is any different than any other job someone has - I personally cannot imagine being a nurse, actuary, grade school teacher, actor, or politician. Each of those has some aspect of the job that to me sounds so miserable and incomprehensible that I marvel anyone does it. Therapists are no different in that it is a job that they for some reason find or found reasonably tolerable to endure as a career, that matches at least some of their skill set and interest that they managed to find a way to get paid for and some satisfaction from.