That's a good question, I think, thought I'm not sure, the answer is people are now far more expensive, adding more people means less money is made. Let's use your dams example. They built a bunch of big dams in the 50s and 60s and each one had more than 10,000 people working on it. They were mostly publicly financed, so more or less not built to make money. They didn't worry about adding a few more (or a few hundred more) people to the job because they were cheap. Now, the people that do this work are really expensive, and adding a few more means the people in charge make less money. Another big thing is probably rules and regulations and laws, far fewer back then. Good? Bad? I dunno.
I don't think it's the cost of people. Technology has done a pretty good job of reducing the cost of simple labor. But technology hasn't done a great job of managing complexity. See, for example, the above jab at the "millions of lines of software code" managing the SR-72. That stands in stark contrast to the "keep it simple stupid" motto attributed to Kelly Johnson, the SR-71's designer.
