Young Girl Making Cookies In The Kitchen

Oils in baked cakes do not behave as soda tends to in the finished result. Rather than evaporating and thickening the mixture, they remain, saturating the bubbles of escaped gases from what little water there might have been in the eggs, if added, and the carbon dioxide released by heating the baking powder. This saturation produces the most texturally attractive feature of the cookie, and indeed all fried foods: crispness saturated with a moisture (namely oil) that does not sink into it.
History
Cookie-like hard wafers have existed for as long as baking is documented, in part because they deal with travel very well, but they were usually not sweet enough to be considered cookies by modern standards.