It's the perfect storm of using higher moisture ingredients like brown sugar and butter - as opposed to granulated sugar and shortening, and baking the chocolate chip bars at a lower temperature, a mere 300° F. All of those things combine to create thick, choclately cookie bars baked to chewy perfection. King Arther Baking (my favorite brand of flour) has a great article about the chemistry of chocolate chip cookies. Every ingredient and your technique all play a role in how cookies turn out.