Casting parts like that is actually incredibly difficult. The iron will want to freeze before filling the mold cavity, since the cross section of a nail is so small. While tedious & time consuming, hand forging a nail is significantly easier than attempting to cast one. Plus, cast iron is much more difficult than forging simply due to the much higher temperatures required. A simple charcoal fire is adequate for forging, the higher temperatures required for casting necessitate specialized equipment. Hence why many village blacksmiths didn't have the capacity to produce cast iron parts.