What they found was that, from 1952-1988, economic growth was responsible for nearly all of U.S. stock market returns, at 92% (table below). For the subsequent 1989-2018 period, economic growth only accounted for 24%, while the increase in shareholders' share of potential profits was responsible for 54%. In short, rather than reinvesting cash flow into their businesses, either in job-creating projects or employee compensation, companies retained this cash or expended it as dividends and share repurchases.
Data source: Greenwald et. al.
PS. Additional commentary on this study can be found here at Retirement Income Journal and here at Barrons.