The amount of security etc. required per site is subject to change. But even a 1GW site can be more cheaply assembled from 300 units coming off an assembly line

The amount of security etc. required per site is subject to change. But even a 1GW site can be more cheaply assembled from 300 units coming off an assembly line than from one unit built in-place. The economics of this were discovered in the 18th century.