Why are all the frames in eyeglass ads fake?

You might not know this unless you work in optical, but nearly all the frames in eyeglass ads are fake. Basically they take a picture of your face and then add the frame later. Most of these ads are extremely well done, but with lesser known companies it becomes embarrassingly obvious (http://scottharriseyewear.com/). But it’s almost impossible to find info about this practice, which is odd because almost all frame companies do it.

So does anybody know where I can find info on this?