I think it is the difference in resources. The question is asked:
If, as an undergraduate, you handed in a paper that had partially been written by someone else, you would be sent down to the Dean's office, read the riot act with regard to academic integrity, and (probably) given an automatic F in the course.
Why doesn't the same thing happen with respect to a PhD thesis?
In Epidemiology the answer is that there would be no PhD theses under this rubric. Data takes too long to collect and analyze. Furthermore, granting agencies are simply unwilling to give massive amounts of grant funding to a PhD student. All of our research is deeply collaborative and single authored work can only happen in areas quite abstracted from reality or for very senior people who have access to data in a way that others do not.