Why is a normal bolt not used in place of a shackle pin?

A normal bolt is not used in place of a shackle pin because shackles are designed to handle specific loads and stresses in a way that a standard bolt cannot. Shackles have a shape and strength tailored for dynamic loading and are made from materials that can withstand the forces involved, while bolts may not provide the same level of safety or reliability. Additionally, the pin of a shackle is typically designed to be easily removable, allowing for quick connections and disconnections, which standard bolts do not facilitate. Using a bolt instead could lead to failure or unsafe conditions.