Female models - why don't they smile?

Sorry, this is not amateur radio.....

Why do most female models look so unbelievably miserable? Almost without exception, the "look" seems to be to appear as miserable as possible and never smile.  Surely, if you want dresses to sell then wear a smile at least! I am sure models used to look nice and smile. Back in the 1960s I never recall models being miserable. Perhaps this is how they are told to look in our politically correct times?  Rubbish I say! Not that my XYL is ever likely buy designer dresses, but surely they'd sell more if you thought the models wearing them actually liked them?