Check out this excellent post on Status by Will Wister on quora.com:

Why don’t fashion models smile?

The reason for this is actually rooted in psychology.

Smiling in primates – and often in humans – is a sign of appeasement. It’s saying “I’m very happy with what you’re doing.” Because of that, smiling when there is little reason to smile can be seen as a sign of compliance. It is thus a sign of low status.

In other words, fashion models don’t smile because it makes them seem lower in status.

That’s exactly how they don’t want to appear. They want to project an image of being high status. They want you to think “I want to be that girl. I want to have the status she has. Maybe if I buy the clothes or makeup she has on I’ll have her status too.