However, I imagine most people wouldn't believe that, and it's probably not true either. Therapists are humans.
Of course, this kind of raises the question: So therapists have their own biases and we're just blinding ourselves to which ones they have? Is that actually an improvement? (Cf. the koan where Sussman says "I am wiring a neural net randomly because I don't want it to have any preconceptions".)
Perhaps the therapist having to pretend she has no background actually kind of helps her disconnect from it. A kind of theatre. (And maybe the brain having a habit of "when in this office talking with patients, don't connect anything to your real life" is useful.) But I would still wonder if it would work better to bring the issue into the open, and say "Well, I do have this background, which might lead me to think x, but I will consciously try to counteract that." Perhaps there are cases where this second strategy is indeed better (if the patient comes in thinking "I expect almost everyone to be prejudiced against me", and the psychiatrist says "Well, you're right, but I have lots of practice in noticing my prejudices and setting them aside, and it is a point of professional pride for me to do that"), but most of the time it's just faster and less distracting to not mention the issue at all.
Therefore, it's easier for the patient not to know the therapist-as-a-person.
The idea is that you can discuss your issues without filtering them through what your perception of the therapist is.
e.g. If you know that your therapist is a Christian mother of three, you might hold back on how your mother in law is pressuring you to have more children for religious reasons and you think its bullshit. The issue the therapist wants to deal with is your relationship with the in-laws, not whether she agrees on a moral basis with the in-laws.
Therapists need to have a very hard line between personal and professional for their own reasons as well. It is not a business where you want to be mixing different parts of your life.
For these reasons it is a reasonable effort to try and remove as much of yourself as possible from the counselling relationship.
It's not that the therapist doesn't exist in context, its that who the therapist is shouldn't influence the way you present to the therapist.