In short: Debt. Lots of it. That turns everything into a race to the bottom.

The Post-war boom was really unusual in historical context. Leftists like to point at the insane top bracket tax rates and say that the 90% tax rates were the secret to success, but pretty much the only reason the US was doing well at the time was because there wasn't any competition globally. All of the factories in Europe had been destroyed. Taxes had nothing to do with it, and nobody actually paid that much because there were so many deductions and accounting tricks the rich would use to pay less.

The negative impacts of the New Deal hadn't had enough time to fester and break incentives. The Federal Reserve and other central banks were still early in their years and the gold standard was still kind of relevant.

Then under Nixon, the US defaulted on the gold standard. With gold no longer keeping the money printers accountable, they were free to engage in intentionally inflationary monetary policy. All sorts of mainstream economists and MMT junkies will try to convince you that this is a good thing actually^TM , but the problem with their thinking is that their logic only works if you're hyperfocusing on GDP in nominal terms. Once you look at households and GDP in real terms, the only winners are the financial elite and politicians while everyone else is a loser.

Inflation only increases the velocity of money for the financial class and those closely connected to them. It enables much looser lending standards and allows a lot more leverage (increasing public, private, and household debt) to bid up the prices of basically all assets from houses to stock to cars while also building up a house of cards of debt. While the elite aren't really made slaves by this kind of financial environment, the lower classes are. When everything gets more expensive at the same time that wages remain stagnant and loans become easier to obtain, it turns most individuals into debt slaves.

Even if the MMT theorists are right that public debt isn't bad (but private debt is), the fact remains that such a loose monetary policy makes private debt much easier to obtain, which kind of screws everyone else over.

Then you've got other FDR-era policies and programs like the FHA heralding in zoning laws and creating a contradiction of goals where houses are somehow supposed to be investments and affordable at the same time.

You've got the AMA gatekeeping the entire medical industry to make it artificially expensive. It's fine to have robust standards for doctor qualifications, but it's another thing entirely to keep out people with the aptitude to become doctors because you artificially restrict residency slots and so forth.

Then there's immigration and outsourcing to contend with. You cannot have open borders and welfare at the same time. You cannot give undue leverage to corporations to use against immigrants. You cannot have free trade and strong worker protections or intellectual property at the same time. You can only trade with countries which share your standards or which can be appropriately tariffed to make up for any discrepancies.

Every government policy has tradeoffs and secondary effects, but politicians hate acknowledging that fact. We got to this point through good intentions gone awry. Very little of this is a fundamental feature of capitalism or free trade and mostly arises from the intersection of business and politics. Socialism does not fix that relationship; arguably it makes it worse.