Electric furnaces introduce carbon using graphite electrodes which is enough for the carbon that gets embedded in the alloy but to convert the iron oxide in raw ore to pure iron in a blast furnace, you need a lot more carbon as a reducing agent to bind to oxygen, preventing it from oxidizing the working material as the metal cools. The coke also serves a dual purpose as a semi-permeable solid that allows the hot gasses to escape from the furnace without collapsing under the weight of the molten metal.