Well i mean poorly in a sporting sense only. They seem good at NFL, NHL , NBA, Nascar, MLB but its only sports that either just them or just them and Canada and/or Russia takes seriously. And its national leagues so they compete within themselves.
But in the other sports that most "western" culture countries play they either dont take it seriously or as some have pointed out just dont care about.
So sports like:
Cricket
Rugby
Formula 1 (pinacle of motorracing)
Netball
Football
Hockey
They really fail at, and since this is the only major sports most of the other countries plays its the only way we can compare them with the rest of the world and in the above mentioned sports they do perform poorly, especially for such a large and wealthy country.
You've just asked and answered your own question. According to your South African / Commonwealth definition of sport (evidenced by ignoring Basketball), they do poorly. According to an American definition of sport, they're mind blowingly good.
Cricket = Baseball, the pinnacle of the sport is MLB, dominiated by American teams of largely American athletes.
Rugby = NFL, as above.
F1 = NASCAR, similar, although F1 is the pinnacle, America is happy with what they have and dominate hugely.
Netball = Women's basketball. I don't know if international competition is the pinnacle or the WNBA, but either way, the Americans dominate. Hats off to them for spotting that basketball is by far the superior sport.
Football, not their sport - a pass time / novelty to all but a few. We'll know they're taking it seriously when serious University programmes are introduced and no doubt they will come on leaps and bounds, although they won't be able to cherry pick as easily as other sports as clubs worldwide already have their claws into players.
Hockey = NHL? If so, much more internationally competitive than you're giving it credit for, but America are decent. Same goes if you look at lacrosse as the equivalent.
Given that America don't set out to be world beaters in any of these sports, I don't see how you can say that they fail, just that they're not very good.
They do seem to be good at individual sports such as:
Tennis
golf
swimming
Track and Field
So still want to see a sport that the USA excels in against other countries. Lets be honest what other countries really take up sports such as Ice Hockey and Basketball in a very serious manner?
Mostly individual sports, competed in by teams of people. The American Davis Cup / Ryder Cup / Swim Team / Track Teams are all up there with the best and probably the best over the history of the sports.
Regarding Sevens i think the sport really still is in its infancy and no team has reached a high performance, and by that i mean even here in South Africa all the boys just aim for 15 man rugby and if they cant get selected there then they move into sevens. So we arent
taking it up so seriously that we are introducing it at school level in the same manner that we do for 15s and i suspect the other rugby powerhouses are the same hence why they arent really dominating sevens either. But i can tell you now that if nations such as England, NZ, Australia, SA etc. make sevens the priority in schools over 15s and take all their best talent there then the USA would not be nearly as competitive as they are now.
So you could say that the Americans are 50 years ahead in the approach that they are taking?
All a long worded way of saying that I don't understand why you think that not dominating sports that they don't try to dominate or care about is evidence that their approach to achieving sporting success isn't awesome. I would suggest looking at what they do do, rather than what they don't do. The number of talented kids from all around the world who end up in the US collegiate system speaks volumes to me, as does the amount of American expertise and facilities that elite athletes from all around the world take advantage of in preference to what's in their own countries.