I don't think that's what causes doctors to be ethical (most of the time). Pros of being a doctor: high pay, you get to help people. People who don't want to help other people will not become doctors.

On the other hand, people who want to harm others, have a higher chance of joining the police force. (Not all police officers, but some.)

And, arguably, the Hippocratic oath is backed up by malpractice suits.