The children of the “right parents” are likely to move in the right circles. If they attend the right college, they are likely to meet and marry individuals who are, themselves, successful. Every lucky step along the way gives more opportunities.
Also, for reasons that I do not need to belabor, it is advantageous for an individual to be especially bright, unusually attractive, talented, and, even tall and strong. None of these qualities guarantees success, but they all help. But put together they will not add up to very much unless certain other qualities of mind and personality are present. In other words, having all these advantages will not prevent some people from botching up; and we all know someone like that.