They do it because it sells, but the sadest part is, it sells because people want it. There's a real and genuine demand for it from the masses. This is what people, the target audience at which these clips are aimed at want and thus the video directors cater to them.
If you think of it from another perpective, would you really want to see ugly chicks in a music video? Or just fixate on something aethetically pleasing?
Personally I don't stomach pop music easily and my genre Heavy Metal and Hard Rock have other themes that make you think which is why I ussually stock up on those music videos. Once upon a time I saw a few Black Metal video clips selling girls in the same way as pop clips do and I was surprised. And then I thought isn't it ironic? Here are bands which sing about Satan and Sex and yet we get surprised when they put provocatively dressed girls in their clips while pop music does it all the time. Which one is more evil?
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~