Because they can. There ARE standards of beauty, despite the wishes of many that there should not be.
That being said, many women are clueless as to objectively evaluating their appearances. Some who are fugly and floppy, for example, still think they are "all that". Others who are really attractive obsess over minor flaws.
As to breasts, to me, firm is preferable to floppy, and larger (but not "excessive") is preferable to non-existent.
Since any surgery is risky, women should carefully consider whether it should really be done or not. You could seek a second opinion by sending me photos for evaluation. :-)