Only the professor is going to read this

A: I think this is the "if I plagiarize in the forest, will anybody hear it?" rationale. When you write for a faculty member, you are trying to earn something by doing work. If it's not your work, you're lying.

In addition, do you know how insulted someone feels being told that they do not matter? (Think of your teachers—few, I hope—who told you that what students think never matters.) Every time I suspect a student has plagiarized, I feel betrayed, a bit like being kicked in the stomach. And do you want me grading your work right after I discover that the previous paper plagiarized? Think of how it might affect my disposition towards students!

Third, how do you feel about individuals or corporations who avoid taxes (or earn subsidies) because they can afford lobbyists or accountants? Now think about the students who work conscientiously, trying to follow the rules. Imagine what they might do, as a group, if they found you had earned a good grade by plagiarizing?

Finally, there is the notion that taking classes is good practice for when your work is read more widely. You should be glad to have the opportunity to practice writing skills when only a few people (such as your teacher) will read what you write. Giving credit to others for their ideas is a gracious and useful skill for life in general.