Because it defeats the ends of education. If students were given credit for work that is not their own, then course grades would be meaningless. A college degree would become a mere sheet of paper and the integrity of the University would be undermined. To protect conscientious students, therefore, and to guarantee the quality of their education, the University assesses heavy penalties against those who plagiarize.
The Rules and Regulations of the Board of Regents of the University of Texas System and the University's Handbook of Operating Procedures provide penalties for plagiarism which range from an "F" grade to dismissal from the University. If these penalties seem severe, remember that your integrity and the integrity of the University itself are at stake. These rules and regulations are available to students from the Dean of Students and the Office of Student Life (SU1.602), where staff are available to assist students in their understanding of the various rules and regulations governing student conduct.
Finally, the University cannot prevent students from plagiarizing, but it can make sure that they know what plagiarism is, what the penalties for it are, and in what jeopardy it places future careers. Hence this statement. Read it carefully. If you do not understand it fully, consult your instructor. And, if you have any doubts about the originality of a paper you have written or a comparable assignment, see your instructor before you turn it in.