On a more serious note I would say it's because for the most part, women above the age of 23 have out-grown the worrysome "do I look good enough" phase and found how they like to express themselfes; physically and mentally. Though if it's physical attraction you're talking about I would again go for the "found the way they like to carry themselfes" road, as most PEOPLE are damn insecure about themselfes through their teens and twenty's., and try all kinds of ugly "trend" styles and ways of walking/acting/looking at you. It's sad but it's part of growing up for most people. I'm just glad most people outgrow the undeasy phase.
Peace.