maybe you should ask single girls WHY she wears makeup. and i’ll even answer your question. if i did not wear makeup, i would not want to look in the mirror. i wear ‘full face’ makeup every day but it varies in heaviness based on the setting.
as hockster613 said, its usually to cover up a problem spot or bring emphasis to one area. ei, eyeliner to brighten eyes or make them look larger than they are. but i agree there is a fine line between looking respectable and presentable and overdone/circus clown. each situation a young women puts on make up, being for work, college, shabbos, wedding etc. requires a different type of application, be it heavier or more neutral. it kinda a sensitivity to know the amount of makeup needed, just like certain styles of dress are ‘tznius’ but those with a sensitivity to tznius won’t wear that style.