so here's the thing. what would you have to gain by telling a doctor or someone that you committed a crime? probably none right? so why do it?
It's not a question of why you would do it, but what would happen if you do? You could also ask why would someone commit murder or molest someone, but it still happens daily all over the world. People do stupid things and reveal secrets and nasty stuff after getting drunk in the bar.
If you talk to a psychologist because you are depressed, the doctor is going to ask you the reason for your depression. Maybe you will put off telling them the exact cause and eventually mention you hurt someone. Since psychologists insist you keep talking, maybe you will mention more details and that's how the information about a crime gets revealed.