We should want to catch and hold students accountable for acts of academic dishonesty -- plagiarism, copying exam answers, etc. However, at my university at least (I don't know how common this is) reporting any such incident entails a significant burden for the instructor. To begin with, there's the several hours of work involved in filing a formal incident report, with multiple copies of the suspected sources attached and the plagiarised sections highlighted, etc. Then one must put aside another afternoon to attend an official "hearing", present a statement, hear the student's defense, and - if they are found guilty - see them suspended from the university for a year (as seems to be the minimum punishment for such infractions).
Hearing of this (and seeing how burdensome the whole process feels for those who are forced to go through it), I can't help but feel extremely relieved that I haven't detected any such academic dishonesty in the work of my own students. When the process is burdensome, it makes instructors want very much not to go through it. That is, it makes instructors want not to discover any evidence of academic dishonesty in their students' work (whether it exists or not). So, in all likelihood, they won't look too hard, and their wish will be granted.
This is obviously a stupid and counterproductive system. Even if minor violations are suspected, there are strong disincentives against pursuing the matter further. (Imagine: the formal procedures are dreaded by the instructor, and the promised punishment seems unreasonably disproportionate anyhow, for what may be a first time offense. So the instructor reasonably decides to do nothing. As does the next one, and the next. And so a serial offender can get away with cheating their way through college.)
If we actually want to prevent cheating, we need to make it easier to enforce discipline. Here's one possibility: let departments deal with it mostly "in house", with an 'F' grade and a simple explanatory note for administrators to attach to the student's internal academic record. Only bring out the big guns -- the 'hearings', etc. -- if (i) the student wishes to appeal, or (ii) prior notes on their internal record reveal that they have become a serial offender. Indeed, even the latter might be dealt with by the appropriate administrative committee unilaterally imposing some punishment (say 1 year suspension for a third minor offense), with instructors only being called to the hearing if necessary -- e.g. if the student wishes to appeal. In short: we need more plea bargains, fewer trials.