On the basis of exactly what evidence
Well, the proliferation of online and physical tutoring services for one thing. This aspect pertains to the gradeschool to high school education of the students. The teachers themselves advice that students get extra study help during the week to help them keep up with homework, research, projects, and allow for lesson clarification. Parents have hired tutors of either type to assist their children. Some teachers, even go so far as to make recommendations for tutors who can help the students. These tutors come in as study buddies but often, for an additional fee, will complete the work for the student as well.
Academic writing has become more advanced for some writers. Those who are now engaged as tutors / study buddies, also do the work for the students when asked to do so. The difference? While it is now more obvious that the student either did not write the work or, had help in developing it, the teachers find it more difficult to prove it. Mostly because the writer is now also tasked with preparing the student in terms of defending his paper should the need arise. Gone are the days, for these advanced academic service writers, of simply writing the paper and not asking questions about how the paper will be used. Since this is a one on one physical or virtual study buddy / writer system, practically an exclusive set up, they now have to add prep work just in case the student is questioned about hiring professional services. They not only write these days, they also have to teach as a part of the tutoring package. If the writer is good, the student will never get caught. Or, if suspected, the student will be able to academically get out of the jam because he was prepared for such instances.
For college students, the university writing center has become a home for on-campus academic writer hiring. The professors know this is happening. However, because the writing center is involved, they opt to not say anything about it. Due to some professors falling on hard times, some of them have also taken to writing academic papers for students. These are students who are not a part of their class roster and, as much as possible, are asking for writing help outside of the subject he teaches. These occurences normally happen for non-tenured professors and never, at IVY league universities.
I am not pointing any fingers at any specific university or professor. What I am saying is that because finances are tough for the educators, they have had to get creative as well and jump into a business they would normally turn their noses up at. They discuss these activities in hushed tones amongst themselves. They know who is in the business and why. They stay quiet because, they just might find themselves in need of the same work in the future. It is better for them to build their bridges rather than build a moat amongst their peers.