One of the most important explanations is being overlooked here.
Income sources include workers, and the rent of land and owned capital. In a primitive society the human workers themselves comprised most of the wealth; a semi-skilled worker in a village of 100 might expect to get 1/100 of the income. But with increasing industrialization, more and more of the total wealth takes the form of invested capital, scarce land, and very skilled specialist workers. It is straightforward to conclude that unskilled laborers “deserve” far less than their proportional share.