I believe it's the nature of girls to want to look beautiful. Boys on the other hand, do not care much about their looks. Every girl wants to look attractive to herself and to other people, which helps them gain more self esteem.
The question you SHOULD be asking is why do guys prefer girls that take the time to enhance their beauty and look attractive for those guys! Women care about their appearance because men care about a women's appearance. You should be happy that women are beautiful creatures to look at, the world would be a boring place without them!