Open book take home exams were where it occurred, not in the actual proctored exams.
Well, OK, I guess. I lived off-campus, so I took most of those exams sitting in the library. I didn't observe any group work or other obvious cheating based on what I saw, but there could have been some in the dorms that I didn't see. Take-home tests are really just 6-hour tests with no restrictions on where you can work at Harvard - you pick your exam up at 10 AM, write for six hours, and turn it in at 4PM. I can't remember more than maybe one class where I kept a test overnight.
Now, I also tried to take in-class exam classes rather than take-home exams or (god forbid) papers whenever possible. So maybe I was just exposed to less of it.