[philipkglass]

That graph only goes back to 1947. If it went back to 1847 (I know, data isn't available) I suspect that we'd see the immediate post-WW II decades as unusual rather than a natural baseline.

Why was the economy so great for median American workers between 1945 and the mid 1970s?

One factor was that the USA was the world's largest manufacturer and running an export surplus. It was relatively easy to run an export surplus and offer good working conditions to the rank-and-file because there was limited competition (self-imposed isolation from the world market economy by Communist states, various market economies still recovering from the war).

Some of the economically glorious post war Golden Age was also built on environmental debt. Businesses faced less regulation on environmental, health, and safety issues and correspondingly spent less on pollution control, worker protection, and compliance documentation. Just don't drink the water or be "unlucky" on the shop floor. Building a $10 million chemical plant in 1965 counts toward that year's economic growth even if it's going to become a $100 million Superfund site by 1985.

Another factor was that there was a huge prolonged bounty from basic and applied research that came out of the physics revolutions from before the war. Nuclear fission, quantum mechanics, and relativity were all known by the 1930s. By 1970 most of the low hanging application fruit had been plucked. There's still plenty of basic research to do, and we still find new applications over time, but we haven't had another physics revolution like QM that just pops out one delightful surprise after another.