Humans are a competitive species with hierarchical societies. Males compete for dominance by using intimidation and strength. Females compete for status with popularity and beauty. You will note that the alpha male gets his pick of beautiful girls, and the beautiful popular girl can get whatever she wants with just a smile.
In modern societies it gets more complicated because males can gain alpha status simply by being wealthy, or famous. Females are no longer limited to popularity and beauty, but they can also compete alongside males for intelligence, leadership, and wealth.
However, for female status in the social hierarchy, beauty still ranks high. So women wear makeup because they want to look and feel as attractive as possible.