As economist Russ Roberts often points out in these inequality discussions, the quintiles that are used to show growth only in the upper class does not refer to the same people over time. When you follow people over time, they generally move up into higher quintiles thus "getting richer."
https://russroberts.medium.com/do-the-rich-capture-all-the-g...
He also talks about "what happened in 1972" is more about demographic shifts due to divorce, single parent homes, and children leaving home sooner: