Manufacturing needs a highly controlled environment. The industry's trend has been towards simplifying the installation process as much as possible. Smaller crew, simpler equipment, less that can go wrong on-site, etc.
You want as few humans off the ground as possible for safety reasons. Someone needs to get to the top of the tower to attach things, but as little as possible.
Most wind farms (on shore) are in fields that the farmer has planted. you want to disturb the crop as little as possible while installing which means the factory will be elsewhere so that you don't need to tear up more crop in manufacturing.
