How can you say tennis players are overpaid? Yes, compared to distance runners. But probably fewer than 150 men and 150 women who play singles can make a living playing tennis. A lot of those really don't make enough given all the sacrifice involved but continue out of hope to one day make the higher rankings.
Like many sports, a few players make bank from endorsements. But it's helpful to be from certain countries. Russian players generally don't make that much from endorsements even though they are well represented at the top of the sport.
Of course, national origin can be overcome by personality and intangible factors. I would guess Iga has more of the latter two because I don't think being from Poland will necessarily get you the big bucks.