Well, for a lot of them it's just habit, nobody's ever told them not to use that sort of language. Around a campus, students think it's "cool" and "adult" to use foul language.
I've known some people who, after years of talking like sailors, try to clean up their speech and find that it's become such a habit, they have real trouble not using it.
When I was teaching freshman English at the University of Houston, part of my first-day talk was the detail that I expected standard English to be spoken in my classroom and to be used on all assignments.
Standard English did not include what was then called "standard black English," or Ebonics, which I forbade along with students' native languages.
When you're in my English class, you speak English.