Because professors are in a position of authority over their students, and part of their job is to evaluate the students. Thus preventing or detecting cheating

Because professors are in a position of authority over their students, and part of their job is to evaluate the students. Thus preventing or detecting cheating (for example, by changing the structure of the assignments, or using various plagiarism-detection tools) is legitimately the professor's responsibility (though not solely theirs; the school and the department are responsible for setting policies, and handling violations after the professor detects them; and of course the individual students bear the ultimately responsibility to not cheat!)