I can tell you have never built off a government design. I have.
You get a design that was held up for two years of government infighting. The technical standards and products made have all subtly changed. The As-built plans furnished by the government were done by people now dead and incorrectly show the size, flow direction and location of utilities. For the technical projects the changes needed to build it useable can cost 10% to 40% of the whole project.
As a contractor I found that demolition of a high pressure fire main serving three other buildings as directed to build my building was often not desired. Instead the added rerouted line and by-pass valves to keep water service in place was preferred but took more time and money. The examples from just project I ran can fill a book.
I once waited three months for the government, their facilities people, the Air Force, and the Corps of Engineers to decide on the chacteristices and exact type of an underground three block long electrical feed they forgot to include in a complex design. We agreed to the 340,000 it woul cost — we worked through the night and weekend to install it and spent agony testing and get it going and 36 hours after we were done they requested we tear it all out and redo it in different manner.
Wasn’t it President Reagan that said an elephant was a mouse built to govt. specs?
And the requirements were all hammered out using the waterfall method which is a very right management implementation style. Has anyone tried the Agile method recently?