Foundation is not nearly as noticeable as other makeup products. You may be seeing a lot more women wearing it than you realize. It was more noticeable back when a lot of brands had literally only 3 shades, but now that some have 50 or more, it matches better and looks more natural than it used to.

Maybe it's because I'm looking for it, but I always notice foundation because the texture on the skin never looks good no matter how smooth it is, and the too-even colouring of the face is distracting.