Why do universities require official transcripts?
Official transcripts have the registrars imprint. He or she is putting his or her reputation behind the accuracy of the transcript. That makes it official. The person it refers to has not made himself look better or worse. This is the result of his work. Before a different university allows credit for work taken elsewhere, he or she needs official verification that it represents earned grades. Otherwise, anybody could make up their grades if they transferred institutions.