Many instructors are concerned that students do not understand what constitutes plagiarism. Therefore, they spend more class time defining it and teaching students how to avoid it than they might otherwise.
Teachers assign projects because they want students to learn something. Plagiarists simply opt out of the learning process and gain no new insights.
Teachers who wish to maintain the academic integrity of their classes should do two things: work to catch plagiarism whenever it occurs and design assignments that are difficult or impossible to plagiarize---both require significant time and energy.
Students are in competition with each other for positions at the next level---college, graduate school or career. Plagiarists who go undetected have an unfair advantage over their honest classmates.
Cheating results in a loss of integrity. Beyond that, plagiarism may cause a student to fail an assignment or course---or even suffer expulsion or the revocation of a degree.