I think because incomes from real estate are based on scarcity rents: you buy the right property at the right time, and you get rich very quick. No-one can dissipate your rents. But if you are in manufacturing, you have to compete not only with your international competitors, but also with copycats and imitators at home. So the rents from successful ideas get dissipated quickly. It's not that the overall gains are not large, and even larger than in successful property investments. It's that the innovators can hold on just to a small share. So real estate creates billionaires; manufacturing creates a middle class.
The U.S. is seeing its manufacturing sector decline as job creation seems to be in the service-producing sector rather than the goods producing-sector. This source notes the total nonfarm employment rose from 121.232 million in January 1997 to 138.265 million last month. But the goods-producing sector fell from 23.619 million in January 1997 to 22.324 million last month, while the service-producing sector saw employment rise from 97.613 million in January 1997 to 115.914 million last month. Over this period, we have also seen an increase in income inequality. When I think of the service economy, I think in terms of bimodal distributions:
Bimodality of the distribution in a sample is often a strong indication that the distribution of the variable in population is not normal. Bimodality of the distribution may provide important information about the nature of the investigated variable (i.e., the measured quality). For example, if the variable represents a reported preference or attitude, then bimodality may indicate a polarization of opinions. Often however, the bimodality may indicate that the sample is not homogenous and the observations come in fact from two or more "overlapping" distributions.
Some service sector jobs require few skills and have intense competition. Think of burger flippers and those who clean hotel rooms. Some service sector jobs require an advanced degree and have all sorts of barriers to entry. Think of doctors and lawyers. So how much of the increase in income inequality comes for the U.S. moving from a manufacturing economy to a service economy?