Why wealth gap has grown despite record-long economic growth

The corporate tax cuts were intended to increase earnings, increase production, and increase capital investments. However most of it was invested in stock buybacks which increased the wealth of those invested in stocks or stock mutual funds. Corporations are faced with uncertainty of material costs, accessible markets, and when to increase production facilities. Corporations will not invest in increased production capacity unless the demand increases for products.

There is no mystery here. It's based on the same principles as compounding interest.