Warren is touching on an important point here. There has been a historic breakdown in the share of income earned by labor vs. capital. The reason for this is the neo-feudal corporatism that has taken hold in this country by controlling the banks, media, and government.
I don't trust Warren but she shouldn't be slammed when she's doing something right. What indeed happened to the earning power of a man's labor, and why? This question leads to profound and uncomfortable truths about our monetary system and related government policies.
The real question is not so much, "why has capital income grown and labor income stagnated?" but rather, "why aren't more people becoming capital owners?" And the answer to that question is because of government.
Even without government interventions the share of labor income would have likely gone down (in relative terms) as automation continues. That wouldn't be a huge problem though, since more and more people would become capitalists.
Also, the whole paradigm is a wrong one. Aggregate income is not a ready-baked cake with capitalism or the government wielding the knife of income distribution. It is simply the aggregate of individual incomes, each of which could be higher or lower without the government.