An S&P 500 index fund has an average annualized rate of return of about 8% over the past 50 years. It is barely beating inflation but if you can accept the risk that you might be in a -10% year at any given time instead of a +20% year then it's the best place to put your money. This is where those profits are going, and that long-term rate of return hasn't moved much.

A traditional pension fund cannot afford that kind of risk without a massive cash buffer and could not have captured that value anyway because it cannot cover its own firm's losses with some other firm's gains. After all, it's not like the same 500 companies are occupying the index as decades ago, and even those that stick there have changed relative positions a lot.

The only thing that could have conceivably replaced traditional pensions and captured all of this value and given it to the workers "fairly" without suppressing the factors that made it possible would have been a sovereign wealth fund run passively but competently by the government, but such a thing is in and of itself a moonshot.