It is all about whether those with the most genetic potential end up deciding to run marathons.
The U.S. probably has several hundred Rupps and Ritzenheims, but most of them don't run. My guess is that in certain parts of Ethiopia and Kenya there is a much greater chance that someone with potential will end up running marathons.
Anyone who runs under 2:10 is a genetic freak. Anyone who runs under 2:05 is a bigger freak. I think that the U.S. probably has more of those genetic freaks than either Kenya or Ethiopia (even if the U.S. has a lower %, it should have more in absolute numbers, because the U.S. is 8 times bigger than Kenya) but most of them probably do something else besides run 140 miles a week.
As has been said many times before, there is a huge financial incentive for the Kenyan and Ethiopian runners. So the talented have every reason to run 140 miles a week and people have every reason to run enough to see if they are talented. In the U.S. that is not true. The best marathon runners make what--maybe $500k a year. In the U.S., if you want to make that money, it makes a lot more sense to get a computer science degree and move to San Francisco than it does run 140 miles a week.
In comparison, I bet the U.S. does a pretty darn good job of identifying football and basketball talent. If you live in Texas and run a fast 40, you will find your way to the football team. There probably is not much missed talent with these sports.
I'm stealing several points from Malcolm Gladwell, David Epstein, and the 500 other threads on this exact topic that have appeared over the years (I'm not complaining; I, like many people who frequent this site, love to make the same arguments over and over again).