[T]he federal government doesn't have much of an incentive to maintain lots of income inequality. Not much fiscal incentive anyway. For the most part, the political incentives swamp the fiscal ones, and unfortunately they aren't very closely balanced. Pursue policies that raise middle class wages, and the effect is so diffuse and so slow that hardly anyone notices. Pursue policies that benefit the rich and you get immediately showered with oceans of campaign contributions. That's mostly what motivates our political economy, I think, not tiny changes in the total tax take based on changes in income inequality.
Free Exchange has further thoughts.