The truth is that cast iron actually doesn’t heat evenly at all, and is in fact quite a poor conductor of heat — which is its main advantage. Once it gets hot, its “high volumetric heat capacity” means it stays hot, making it better at cooking. If you take the time to preheat your skillet for about ten minutes before plopping your steak or eggs into it, then your pan won’t be cooled down by the addition of food, and end up cooking that food more evenly.
Source: What is the point of a cast iron skillet?