After WW2, Europe and Asia were rubble, and needed to rebuild. And the systems, structures, and customs that had existed pre-war had fallen apart. They all needed, simultaneously, to rebuild and modernize.

To do that, they needed cars, machinery, home goods, electronics, etc. They had the labor to produce those things, but not the infrastructure. It takes time to build factories, and a skilled labor pool, and a logistics network, and so on.

So where did you go to get the goods & services you needed to rebuild? There was really only one option. The US was exporting cars, factory equipment, heavy machinery, steel, radio, coca cola, etc. They had an intact industrial plant, and had lost (relatively speaking) very few working-age men in the war. That helped them ramp up quickly with internal demand (fed by pent-up war wages).

For reasons laid out above, it wasn't practical to move factories overseas, or outsource parts, or automate. So workers in the areas with factories were in very high demand, and wages went way, way up in those areas. That had knock-on effects: America was just beginning to import oil in large quantities, so American coal & oil was suddenly in high demand. Same with mining, logging, etc. That caused a general boom--specifically favoring labor.

It wasn't because the rest of the world was poor that the American middle class was rich. It was because the rest of the world was developing, and America had a near-monopoly on the means of doing it. What's happened in the meantime is just that the US has lost that monopoly. Now American workers face relatively fair competition. This has been a huge net positive for the world, with cheaper goods and higher wages pretty much across the board...except for American workers.

Where is this wealth coming from though? The other countries aren't producing anything, everything is being produced by America. America would have to produce everything both for the domestic market and the entire rest of the world. And consequently why does this wealth suddenly disappear once the rest of the world catches up. You are talking about demand, but don't mention supply.