A big part of it is wage stagnation too. The average American hasnt had a meaningful raise since 1973, that's half a century. Not good. Growth of the labor supply and repeal of trade protections for industries that employ low-skill workers have caused it.

This metric is comparing returns to capital to total returns to labor, not the median returns to labor. Median wages haven't been doing too well recently due to rising inequality but gains by the best paid professionals would be enough to make up for that on this metric except for rising housing costs.

I dont see how a low median return plays no role in total returns. If the median were higher surely the total would be higher no?

All things being equal, raising the median wage income raises the mean wage income. But the mean wage income can also rise form things that don't raise the mean income, like people in the top quartile being paid more. And you get the total wages paid out by multiplying the number of wage earners by the mean wage, not the median wage.