Why does the Macbook Pro 13" model lack a discrete graphics card?

I enjoy smaller laptops (i.e. I own the macbook air 11.6" model) but am interested in a laptop with a more powerful graphics card or a graphics card at all. I just wonder why Apple doesn't include a discrete graphics card in the 13" model of its Macbook Pro line. Is it simply a space issue within the chassis? Besides including the super drive I'm not even sure why someone would choose the 13" Macbook Pro over the 13" Macbook Air. Thanks for your Insights.