- Some women, especially in earlier decades, were taught by their mothers that this is what a lady does in order to be presentable to the world. (Interestingly, before about the 1920s, makeup was usually considered the mark of a woman of questionable morals, so this is a relatively recent idea.)
- Some women have been so conditioned by the images of women they see in magazines, movies and on television that they feel inadequate without makeup. This is enforced by such things as articles in tabloids about “Celebrities Without Makeup!” where extremely unflattering pictures of women’s naked faces are splashed all over the cover, complete with black bars over the eyes as if they were criminal suspects who could not yet be identified. Or by other people (men and women) who are so used to seeing women wearing makeup that if a woman appears without it they ask her if she is sick. People who do this are often men who claim they like “natural” women without makeup, but don’t realize that the pictures they see of such “natural” women have been produced by hours of work with neutral cosmetics, plus further hours of Photoshop.
- Some women do it because they like the way they look in makeup. It gives them confidence to know that they do it well and pleasure to look the way they have decided to look. These women can often also go out without makeup and not really care.
The main point is, do they do it because they feel ugly and inadequate without it, or because they like the way the look with it? Is is a requirement, or a choice?
(Interestingly, this answer is reminding me of the varied reactions to Muslim women wearing the hijab, or headscarf. Many Westerners see it as a sign of oppression, but most of the women you speak to will say they do it for themselves, for reasons of self-respect, religious identity and to be taken seriously in professional situations, rather than being judged on their looks.)