Because the Americans simply didn't need castles. Firstly, the USA was founded a long time after the castle had become obsolete. Secondly, the castles are structures built for defence reasons. The last serious attempt of a foreign power to invade the continental USA was made by the British during the War of 1812.

Castles were not built in America primarily because the feudal system, which necessitated their construction in Europe, never existed in the Americas during the colonial period. In medieval Europe, castles were fortified residences for nobility that served as military strongholds, administrative centers, and symbols of territorial control in a fractured landscape of warring lords. By the time Europeans began large-scale colonization of North America in the 17th century, military technology had advanced to the age of gunpowder and cannons, rendering traditional stone castles obsolete. Instead of high-walled keeps, colonists built star forts and coastal fortifications (like the Castillo de San Marcos) designed to deflect cannon fire. Furthermore, the socio-economic structure of the New World focused on plantations and trade rather than the manorialism of the Middle Ages. While there are "castles" in America today, such as Hearst Castle or Boldt Castle, these are Gilded Age mansions built for aesthetic prestige rather than defensive warfare.