Stonehenge was already sacred to the celts when the romans arrived in england. why did the myth-makers relate arthur to kings who are supposedly buried at stonehenge?

Geoffrey of Monmouth, bishop of St. Aspah  (1100-1154), is credited with spreading a long-held belief that relates the megalithic monument to Merlin, the famous magician friend, and protector of King Arthur. He says that Stonehenge stones were brought from Ireland to the plains of Wiltshire with the help of some 'artifacts' to commemorate a mass burial of Bretons.

This Welsh Cleric relates this magnificent Stonehenge monument to the most famous druid/wizard, Merlin, and King Arthur. The legends speak even of the king's father, Uther Pendragon.

In one of his works Geoffrey of Monmouth, tells that the magician Merlin advised King Arthur to move from Killaraus, Ireland, the stone monument known as "the circle of giants" to its current location. The idea was to honor for eternity the memory of 460 noblemen killed by the Saxons and buried in the place. The envoys of the king could not dismantle the monument and it had to be Merlin himself who, magically, moved the stones to where they are today keeping its form.

d. it adversely affected american productivity