But demographics have changed dramatically since 1970. For example, immigrants make up almost 3x as large a fraction of the population (from 5% to almost 15%) than in 1970. Obviously, if you add a large number of (typically) economically disadvantaged people to the population, income inequality is going to go up, even if there is no change in incomes for everyone else: https://www.migrationpolicy.org/sites/default/files/source_i....
Likewise, peoples’ incomes increase with age. The population overall has grayed significantly. Again, even if wealth inequality wasn’t increasing for people of any given age, changing the age distribution of the population will affect income inequality.
Finally, a larger fraction of the population now comprises historically disadvantaged minorities. While the income gap between races in the US is a huge problem, it’s a different problem than “income inequality” genetically, deserving more targeted solutions. (And even that income gap is hard to analyze, because the ages of people of different races in the US differ quite dramatically). The median age of non-Hispanic whites is 42. The median age of Hispanic people is 28. The median age of African Americans is 33: https://www.pewresearch.org/fact-tank/2016/07/07/biggest-sha...).
Income inequality probably is going up. But I’ve never seen a study that actually holds demographic changes constant so the result reflects peoples’ perception of what income inequality means.