I read a passage related to this, in a book called 5000 years of trade. Countries can be categorized as rich/scarce in capital, land or labour. Those countries that are rich in say capital, tend to enforce protections on capital while relaxing the policies for land and labour. So coming back to this article, I would say that US being rich in capital has strictly enforced and nurtured conditions on its society by which capital has decent return through investment into student, house debt etc., while meaningly deteriorating the conditions for labour (wages). This is the reason why it is incredibly difficult to become rich nowadays in US through wages alone..