I blame society.

No, seriously, I kind of do. Lots of people end up in therapy because they have persistently received a message that they are worthless and should not have power over their own lives. This message is delivered in any number of ways: abusive relationships, traumas, marginalization of minorities/the sick/the mentally ill, etc.

So I think some give it away because that is what they are used to: someone else having the control and the choice, and a therapist seems like an authority figure that can be trusted. And, since therapy like any relationship is susceptible to abuse, some therapists actively encourage clients in giving away their own agency to the therapist, to feed his or her ego or need for control or whatever issue the therapist has.