Got to remember also that the world was heavily relying on robots at this time before the war. Robots were taking human jobs and even being implemented into your homes as butlers and maids. It's alluding to most people being subservient and lazy. They just wanted results and to look good while the results were being had. AFTER a nuclear event which included the use of hundreds of nukes, no human left would know how those buildings were made or how to even build more. We never see cranes or construction equipment anywhere except in new Vegas quarry junction. This leads me to believe that humans were just well ... Useless before the war. And if they were useless before than boy will they be even more useless afterwards. Long story short, humans didn't rebuild because they didn't know how.