There are usually policies to give guidance and mentorship on 'academic conduct' for minor and first-time offenders; but if the cheating was so extensive, then that's a serious dereliction on the examiner's part.

I can understand why it happens though. For the professor to take it further, they'd have to escalate it within the school, involving a whole load of bureaucracy, meetings, production of evidence, and likely a formal hearing with the student, etc. Unfortunately, there's often no incentive or institutional support for professors to do any of this.