Ever look through a fashion magazine? How come in every ad you see there is a model, or numerous models, in which none of them are smiling? I tell you there is a reason for this. It falls into the category of perception..... how you perceive the advertisement... how you perceive the product? The photographers and clients want it to be this way. This is all part of their strategy. Of course it all depends on the product being sold, let me explain.
All magazines have advertisements in them and advertisements have a big job to do. The advertisement must sell the product and make it seem as though this product is extremely desirable; and when it comes to fashion and style, sexiness and confidence are desirable. Simply put, sex appeal and confidence sell products. Smiling typically does not create overwhelming sex appeal and or confidence. Smiling creates an innocent girl next door persona. That is not what these types of advertisements are after.
Now it is important to note that straight-faced models are often found in fashion and style magazines using sex appeal and confidence as their marketing strategy. But if you take a look in say, a home and garden magazine, you may see plenty of smiling models. So it really depends on what magazines you are reading. You may see plenty of smiling models or none, depending on the magazines you read most often.
To truly answer the question of why models don't smile it is because straight face models with sexy, intent eyes create sex appeal and displays confidence better than a cute smile. It is all about the audience that the product is intended for as well as the perception that the product gives off to this audience. Models smile in ads all the time; these ads just don't scream for people's attention as other ads might. Ironically, the ads that people remember the most are the provocative ads displaying much sex appeal and confidence. When it comes to modeling and advertising, it is all about the feeling that the model makes the viewer feel.