1) The distribution of income has been different over time. 2) If it had been pegged at 70's levels, and wealth generation had been unaffected, incomes would have grown more. 3) But this hypothetical didn't happen. Ergo 4) You're being robbed by the enemies of the proletariat.
Also, the data on offer strike me as odd. There has been far more than 8% wage growth since the 70s. This Pew Trends article, which controls for factors like household size, indicates that household income has risen 49% since 1970.[0] Note that incomes have risen significantly since 1990, even as the number of dual income households has remained flat during that period. [1]
[0] https://www.pewsocialtrends.org/2020/01/09/trends-in-income-... [1] https://www.pewresearch.org/ft_dual-income-households-1960-2...