The spoils wrote:
Rupp, Webb, and Cain are said to come from affluent families. Most tennis and golf players are from relatively wealthy families.
I think there may be a flaw in your logic. Many people better themselves in spite of family money. And playing a sport for a living probably doesn't qualify as bettering.
Depends how you define affluent
I would say most distance runners came from above average (in USA) public high schools and/or private schools. Parents usually upper-middle class and have saved to put kids to college
Poorer schools tend to have track teams where there are zero distance runners but perhaps great sprinters, baseball, football and basketball players