Is it really that hard or asking too much for this lazy department to tweak the code so that it's different from prior years?
The real issue is that students keep this stuff and pass it down year to year.
I remember taking ECE 451 at Illinois and someone had taken photographs of the solutions that were posted on the wall for students to look it.
Really only enough to fuck your spot on the curve if you got zeros on them.
Exams were like 80%+.
Students can, should, and frequently do, use past assignments as a learning resource.
At the beginning stages of a music theory course, you are learning the very basic rules. How did Bach's music work? That's where pretty much everyone starts. And you learn how his music worked by finishing his stuff. The early exercises (which I think are analogous here to having some code or library provided) are basically, "Here's the first part of a piece of music by Bach. Finish it in the style of Bach."
Then you go from there, gradually, with less and less hand-holding.
Is it possible to plagiarize these things? Of course! You can just look up the piece and finish it the way that Bach did.
There are some students who did things like that. The professors never bothered to say anything. They didn't have to. Because by the end of the class, you are all on your own. "Write a short piece of music in the style of Bach with the following features: . . . ."