Client and therapist share power in the relationship - right up to the point the client becomes emotionally invested, at which point the power is immediately transferred to the therapist.

The only power the client ever has is whether or not to remain a client. The therapist gets to choose time, place, fee, how much support they will offer, whether or not you can call them outside sessions, whether or not they will take off for two months on vacation, etc, etc, etc. More importantly, the therapist is never, ever as dependent on the client as the client is on the therapist if emotions come into the picture.

As long as therapy is an emotionless business transaction, it's an egalitarian relationship. But therapy is very rarely that. Most people become attached to their therapists on some level at some point. (Not all, but most.) And as soon as that happens, they're screwed. Because emotional transactions take place in a completely different realm than financial / business ones do.

Any client experiencing attachment, transference, regression, etc, is at a disadvantage. And therapists know this, which is why there are a zillion rules around sexual contact, outside relationships, etc. A therapist who has sex with a client is abusing their power - so it's not even really a question of where the power lies, because it's pretty much enshrined in the basis of therapy.

Most people don't like being at a disadvantage power-wise, which means they develop all sorts of coping strategies to deal with it. They can try to distance themselves and refuse attachment, or deny that the attachment once it exists, actually does exist, or they can simply suffer through it (which is what most people tend to end up doing, and as long as the therapist doesn't screw you over, you'll be okay.)

The power imbalance is my absolute least favorite part of therapy. I could, of course, withdraw and hold myself at a distance, but for me that would completely defeat the purpose of going in the first place, so it's a catch-22.