High earning parents are able to pay for prep schools, tutoring, materials. Those children get into elite schools. Those graduates of elites schools go on to elite, well-paid jobs.

"On graduation, many members of America’s future elite will head for the law firms, banks and consultancies where starting salaries are highest."

Here are some more explicit articles on the phenomenon:

http://www.theatlantic.com/business/archive/2015/07/rich-peo...

http://www.telegraph.co.uk/news/uknews/11763836/Wealthy-pare...

http://www.nytimes.com/2015/12/18/upshot/rich-children-and-p...

If you'd like to read a study on it ("Poor Little Rich Kids? The Determinants of the Intergenerational Transmission of Wealth"), you can do so here:

http://ftp.iza.org/dp9227.pdf

Here's another from Pew:

http://www.pewtrusts.org/~/media/assets/2015/07/fsm-irs-repo...

If you want more information, google "social mobility united states", "intergenerational mobility united states", and read "Twilight of the Elites" and "The Son also Rises."