In addition they would be ridiculously expensive to manufacture if the fins were only on the outside. Normally cookware is made on a lathe (http://youtu.be/8uuFWzkRcAg?t=1m12s) or a press from sheet metal. So, if you have areas that are thicker than others you'd need to use an entirely different method of manufacturing them... like casting or welding on the "fins" after the rest of the pot/pan was shaped.