2. Raising the question why would a profession need to argue it's techniques are effective?
The obvious answer being that limited effectiveness of psychotherapy is one of the criticisms leveled against it.
When all disorders are lumped together, roughly 2/3's of people with mental illness show improvement over several years even without treatment (which obviously isn't to say that all mental illnesses improve in this way). To be shown effective specific psychotherapies must achieve a statistically valid outcomes for illnesses in some manner that is either better, faster, or more durable than the 'no treatment' recovery rate.
In "The Great Psychotherapy Debate", Bruce Wampold (psychologist U Wisconsin) found, also using meta-analysis, that while psychotherapy enhanced patients' sense of well-being, that outcome significantly depended upon the therapists' personality and the therapists' confidence in the therapy they were delivering--issues independent of type of psychotherapy but not independent of patient perception and receptivity to the therapist's treatment.
Setting that debate aside, the talk-therapists want the APA to endorse the effectiveness of talk-therapy compared to psycho-pharmaceuticals. Their careers, and perhaps the mental wellness of millions, depend on utilization of talk-therapies.
But, regardless of potential effectiveness of psychotherapy, it's no surprise to me that Americans willingly seek psycho-pharmaceuticals for mental illness. Americans perceive pharmaceuticals as scientific solutions to real medical problems. The mentally ill, and most of their care givers, want mental illnesses to be seen as real medical problems.
In American society the mentally ill are highly stigmatized and consequently quickly put at social and economic disadvantage. When mental illness is framed as a consequence of chemical imbalances in the brain, that can be treated with drugs, the mental illness is transformed into a neurophysiology condition. Such a condition is less likely to be seen as the patient's fault. By contrast psychotherapy leaves open to social perception the possibility of the role of personal culpability in mental illness. Talk-therapy, rational emotive therapy, skills training, etc. imply the mental illness is a consequence of a personal shortcoming in appropriate cognition, or emotional/behavioral self-control.