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.

Oh interesting! TIL, thanks!