Why is there such variability in care quality in psychotherapy?

Variability in care. Psychotherapy requires a relationship between humans -- not machines. Humans (including therapists) are variable in their skills and interests. Basic competencies must be assured through credentialing, but beyond that things may vary. You need to find a therapist with expertise in the areas you want help with. Other issues include insurance restrictions on timing and frequency of sessions.
Psychotherapy. In the field of mental health it is very important what you know but more important who you are. Best.
Poor regulation. The mental health professions are poorly regulated. Many people who provide "psychotherapy" have little training. In addition to psychologists (who hold doctorate degrees), there are social workers, counselors, marriage and family therapists, etc. In many states, *anyone* can hang a shingle and call themselves a psychotherapist-- the word has no legal or professional meaning. Check credentials!
Many reasons. Unlike medication or surgical treatments, where you can obtain objective, quantitative measurements, there are no such criteria in psychotherapy. Secondly, psychotherapy is not patented, so there is no company to make money from marketing it, so no incentive for nationwide, extensive research. Thirdly, psychotherapy may be offered by many people with very varied backgrounds and training.
Money. My colleagues may disagree with me on this, but unfortunately, it boils down to money. They were clinicians up there with far less training then other clinicians and those with less training are paid less money. That's the way the insurance company save money. However, those with less training often have less ability to do therapy, unfortunately. Those who have more training can offer better care.