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.
That reminds me of the guy who tried to protect his fingers from the pedestal grinder by using the sleeve of his hoodie instead of walking 20 feet to the tool room to get some gloves. His sleeve got caught at torn completely off. Fortunately he wasn't hurt and banned from the metal shop.