Some of these guys haven’t thrown it all away - Junior Bridgeman had a 12 year NBA career mostly as a sixth man, he and three other guys were traded to Milwaukee for Kareem in 1975. He currently owns over 100 Wendy’s restaurants and is worth over 400 million.
Part of it was luck. But Ty Cobb was baseball’s first millionaire. Being from Georgia, he invested in Coca-Cola early. Then, playing in Detroit, he got in early in the auto industry.