Inequality is intentional.
The primary goal of economic policy since the 1980s was to let the rich accumulate great capital in the belief that it would trickle down. This is often referred to as Reaganomics. This policy has done great to let the rich accumulate more wealth than ever.
In most cases, periods of economic growth have resulted in greater equality, as in the 1950-70s. Over the past 30 years, however, the benefits of a growing economy have been concentrated in an extremely narrow elite at the very top. We haven't seen this happen since the 1920s.