So much information, so little time. It is tempting to go online and do your research from the easily found sites. The information is just sitting there, ready to be copied or modified for your own use. Sometimes you can even find essays that have already been written.
There is an old saying: "There is no such thing as free lunch." This can be applied to the easily-found information on the internet. Here is an example. There is nothing to prevent me from writing an article that "proves" that Abraham Lincoln was an alien from the planet Mars. While this is an extreme example, I could just as easily write an article that sounded quite academic and fill it with quotes I made up and that I claimed were things that Lincoln said. I could invent references. While this is an extreme example, there are a lot of essays that are full of mistakes. (If not, everyone would be getting A+ on their papers.) This is the main problem with internet research. It is often not much more than one person's misinformed opinion.
Misinformation can be spread and sometimes overtake the facts of the matter. This is often the case with gossip and urban myths. If people use factually weak information from the internet, not only are they writing inaccurate papers, they add credibility to information that is wrong by trying to bring it into the academic world. In the academic world, the emphasis is on legitimate research that appears in "peer-reviewed" journals. This means that the research is not just blindly accepted but is checked and maybe even improved by other academics who are also experts in the field.
The related problem is that if you can find this information on the internet, so can everyone else who can run a Google search. Imagine the professor reading essay after essay that refers to the erroneous quotes that have been attributed to Abraham Lincoln. The lack of originality would more than likely annoy the person reading the essays and chances are all of the essays would come across as having been written by people as a group. This smacks of lack of originality and the professor would then look to the expert opinions and realize just how easy a road the students took to bat off these essays. Maybe the essays would not be failed - it is not exactly plagiarism but it is definitely a good sign of a mind that really is not all that inquiring.
Even worse, if a number of students in the same class work on a similar project and most of them use the same technique and find the same information, the similarities could be sufficient to make the papers look plagiarized from each other. After all, if six papers use quotes and information that turn out to be made up, a real case could be made by the university that five of the papers were plagiarized from the sixth. Otherwise, why would they all have identical errors?
As for essays that have already been written, just think about how many students have already turned in these essays or variations on these essays. The academic life is full of challenges without setting off warning signals by submitting work that is inaccurate or very similar to essays that the professor has already seen.