You are missing the time it takes to reach the construction site
SmokeDragon. You could consider Build Power a third resource along Metal and Energy. Any time a commander or constructor is not building something, you are wasting Build Power. Moving constructors and commanders, essentially cost Build Power.
In order to maximize the use of Build Power, you want the constructors to move as little as possible.
Imagine having 10 constructors going around the whole map, building Metal Extractors 1 by 1. Some of the constructors will not even reach the build-site before the Metal Extractor is finished and they have to move on to the next build-site.
Splitting up constructors wastes much less Build Power.