It was true for decades before the mid 70s when productivity and wages decoupled. The reason why isn't clear but there was certainly the expectation that wages would increase for half a century in the US.
I think private equity has a large role to play in this. It's sad to me that people point the finger at innovative companies like Amazon, when the real culprits are people who just finance and gut medium size companies you've never heard of in order to eat up the margin that was previously going to workers and extract it for themselves and their LPs.