Earlier quoted context omitted.
Instructors are often unbelievably naive about this, saying things to the effect of "Cheaters are really obvious, so there can't be that much of it." No, the cheaters that you catch are really obvious.
I occasionally teach a college course as an adjunct (fun, and a little extra spending money). I don't know how wide spread cheating is, though if it were extremely common then students wouldn't struggle so much with some of the material. But personally, while I have an "instant F" policy if caught, I didn't really care: My philosophy and that of many other instructors is not one of indignant, "surely I'd know!" about…
I mean, it is pretty unfair to the kids who actually do honest work. if enough people cheat and get near perfect scores, it makes it hard for the instructor to understand what a reasonable two week project is. grades are also (unfortunately) pretty important for internships.
on the other hand, sometimes people who cheat are really bad at it. in one of the courses I TA'd, there were two groups of 3-4 students who would always sit together in labs. they would always turn in nearly identical labs and homeworks, including all the same mistakes! it looked like the only differences came from transcription errors. I never bothered to report them because they failed all the assignments anyway.