With more houses having parts far more efficiently assembled in a factory and erected on site, how does this compare? Plumbing, windows, and electrics still ha

With more houses having parts far more efficiently assembled in a factory and erected on site, how does this compare? Plumbing, windows, and electrics still have to be put in which will require the workers to be on site, along with tools and test equipment. Just seems odd since they’re building pre-fab house factories to make things more efficient.