Because it is, or can be when over-used, trite. Don't people tend to be more interesting the more unbalanced they are? Also, if there is too much symmetry, the eye (or ear, or mind) is not drawn anyplace in particular; everywhere you turn, you are at a place that you could think of as an artistic 'center of gravity'. I think the sense of balance and composition in any art should draw you in, and not just let you slip freely.