They predicted that companies will be able to extract more value from workers in the future. In other words, they think that the top 1% will continue to get relatively richer at a faster rate, as long as that is true profits can improve without the economy improving.
The 2% economic growth rate is an average. It might be the case that the average worker's real wage grows 0% (this is basically true [1]), while corporate profits, which make up ~10% of the U.S. economy [2], grow 20%/year.
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This is the best answer I've seen: https://economics.stackexchange.com/a/21617/7263