This is much more meme than truth, and it's frustrating how many people just take this kind of statement as a prior. Here's a FRED chart of median personal income as a fraction of per capita GDP: https://fred.stlouisfed.org/graph/?g=1b46r
Indeed it has been trending downward, but... not by a lot. In the 80's it was 61% or so (so the median person got 61% of their "fair share" if the GDP was shared equally). Right now it's about 53%, up from a minimum in 2013 of about 52%.
And of course in exchange real per capita income has grown wildly (up more than 50%) in that same 40 years. If you told someone in 1980 that you could get a 50% raise, but only if the people above you on the ladder were allowed to get a 65% raise, would you have rejected it?