Tenure is nothing less than a lifetime job guarantee.
It means professors don't need to work hard or care about students. Tenure keeps new, fresh talent from moving up the ladder and makes it hard for colleges to meet the changing needs of their students. You may well have heard such attacks on tenure and college faculty. After all, people write books and get quoted in the press grinding this ax. You may have memories of a teacher who didn't seem to keep up with his or her subject or care very much about his or her students.