> I don't know why we always go back to income inequality

Because it's pretty well established that outside of the most abject poverty, relative deprivation is a more significant factor driving experienced disutility than absolute material condition, contrary to the naive assumptions many people make that increasing individual absolute material condition increases individual utility independent of external context.

There are, of course, people who don't believe it works that way, people who don't want it to work that way, and people who don't want other people to believe it works that way because such belief threatens their own advantaged position.