At the very end of the 1970s the US economy changed. An economic ideology called “neoliberalism” took hold. That was the idea that markets are a better way to decide things, not voters. The idea that “taxes on the rich hurt the economy” was blasted everywhere. Business does things more efficiently than government. Etc. (Tell me if you haven’t heard these things.)
So big changes were made in the way the US economy worked. Taxes on the rich and corporations were dramatically slashed. Companies were “unleashed” through deregulation. Unions were broken. Pensions disappeared. Etc.
Results appeared quickly. All of a suddent the gains from the economy stopped being shared, and increasingly went to the top few. Over the years I have posted versions of this chart:

Where The Money Went
A study from the Rand Corporation (a research organization) shows were the gains from the economy – as shown in that chart above – went. Rand’s Trends in Income From 1975 to 2018 concludes that around $79 TRILLION that would have gone to working people instead went to the top 1%. That was 2018, seven years ago. Worse now.