There was still a lot of work needed to be done with those. A foundation had to be built to spec and a chimney built. Plumbing and electrical work was needed after the rough-in and then of course all the finishing work. What you were buying was a design and the materials.
Saying that, in the early 20th century there wasn't too much to the electrical and the house probably had basic plumbing for a single bathroom and a kitchen sink plus radiators and a boiler that was probably onsite oil fed. A modern home has far more to consider including insulation, far more electrical and plumbing, HVAC, etc.
Don't forget the even earlier: https://en.wikipedia.org/wiki/Lustron_house