I think part of it is that a lot of women don't realize that foundation has improved and doesn't have to look like a mask anymore.
There's a young girl I see at my college regularly that has great skin and doesn't wear anything but foundation and powder--but the thing is, it LOOKS like foundation and powder. And it calls attention to itself. Looking at her skin, I bet she doesn't even need it, but if she IS going to wear it, she kind of needs to balance it out with eyes and lips. It looks unfinished and strange.