because women are trained from a very young age to believe that they have to wear makeup. and few are taught the proper way to wear makeup.
the point is to *not* look like you're wearing it.
i remember being a kid, and having my mom tell me i should start wearing makeup... i was a total tomboy, and still. i remember being required to wear it on school picture days...
when makeup's done right, it can increase the beauty factor, but once again, few girls are taught the right way.