It's easy, fast and effective. I do it because I work a full-time job. It's quick and easy. All you have to do is change a couple words and put your name on it. It is quite tempting and many students are willing to take the risk for an easy grade.
Many students feel pressure to get that A. Instead of working for it, they find it on the Internet.
Many students feel they won't get caught because the resources of the Internet are huge. The chances of their teacher finding their source is tiny. Either they're lazy, too dumb to do it themselves or don't have the time. I would rather take an F in something that was mine than an A that wasn't mine. I used the info from the site to write my paper but forgot to cite it.