That's a myth. Most people are rich because their parents were. Most of the rest is rich because they were/are lawyers or medical doctors. (For the super-rich, business and finance becomes more important. But for your average "middle-class rich", lawyers and doctors just dwarf their numbers. Business also has highly unequal outcomes).

As to why this myth persists, I guess it goes well with American puritanical roots. Many possibly don't belief in a strict correlation, if not causality, of everyone's financial situation and their individual moral character, but still promote the idea. See also: prosperity gospel, Ayn Rand.