You realise they don't just go into the surgery on a whim right? No good surgeon will perform a gender reassignment without knowing that the patient HAS already undergone significant amounts of therapy.
In the strictest of terms, you are right, the operation won't make someone a woman, but again, what's wrong with it if it means someone can lead a happier life.
Why does it concern you so much?
Also, where are you getting your numbers about the "amount who still kill themselves or have severe depression afterwards"?
Genuine question