I think an interesting question to throw into the mix is why Andy Roddick decided to retire. Sure, 30 was a pretty "normal" age to retire at, but we are seeing Federer having relative success right through now, with always retirement in the horizon but he just keeps coming back for more. Why? What psychologically or economically is different from what Roddick might have looked forward to?
I think it has a lot to do with opportunity cost. If you are famous enough to press palms of rich people, you can follow the example of our former US Presidents, and appropriate the goodwill of your past fame into private wealth... tax free... for just being "available" to peddle influence and talk and act like a hot shot.
There is another reason why US women have success but US men have not. It has to do with race. But it's not what you think. There is a huge amount of risk associated with kids sports. This is because the way higher education works in the USA. You can't just take the BAC and get into college. You have to build a "portfolio" of crap, even to get into a decent public college. When you are just playing on the courts early on, you are sacrificing "value" in building that portfolio if you don't pan out. And statistically speaking, it won't pan out for the majority of kids. So what demographic can afford to slack off on their grades, skip the Eagle Scouts, not bother with school sports or character oriented activities? Well, here's a clue... when you check a particular box on your college applications in the USA you earn a VERY big bump for what would otherwise be "meh" grades, SAT scores, etc. That group has been handed a risk subsidy so that pursuing "fame" and "fortune" in the form of sports and entertainment (including music, fashion, etc.) by the cabal disguising itself as the trustees of the budding intellects of America. Now, why are the African American women able to figure this out, while African American men don't seem to care? Well, that's where it gets sport specific. Once America had Tiger, it became "conventional wisdom" that a new wave of black golfers was going to dominate. It didn't happen. And Tiger really didn't set himself up as an ambasador of Black male culture -- because Black male culture isn't really a consumer culture... at least not the kind of consumption that moves merch. So Tiger shrugged and moved on to peddline merch to white dudes, and stopped being a black inspiration. But in tennis, it was the Williams Sisters, and it turns out black FEMALE culture is a great fit for merchandizing everything from cosmetics to tennis outfits for people who will never step on a court. And in a largely color blind consumer society, they are able to pretty much seamlessly peddle merch to Caucasian women & girls too. That has been a beacon for African American girls to get in the women's game of tennis. When you see more African American boys on the tennis courts instead of the basketball courts, that's when the American mens game will get back to some level of matching the rest of the world elite men.