I'm sure there are lots of other reasons, too.

-Therapists work with people with obsessive or abusive tendencies, and those are not always visible at the beginning. They're protecting themselves and their families.

-The therapist talking about themselves a lot on session is unfair since the client is paying (and usually paying a lot) for the time.

-The development of transference like The Well says.

Many therapists adjust the amount of disclosure on a case-by-case basis.