I have had the rare privilege to see up close examples of how at several US universities, when professors are presented with irrefutable proof that a student has cheated (well beyond any reasonable doubt) the professor will most often do nothing. In the best case they will meet with the student and give them a stern talking to.

The whole system is set up to disincentivize any effort to actually hold students accountable for cheating in a significant way (fail assignment, fail course, expulsion, etc.)

When we read about cases of students being held accountable it's generally the exception not the rule.