That's because Americans (relative to their population) suck. You have to remember, guys like Geb and Bekele are probably the most talented distance runners the world has ever seen. Ethiopia and East Africa in general oozes with talent, after countless centuries of conditions that have favoured long distance runners.
And did you ever read that article showing a higher O2 bonding rate for Ethiopian hemoglobin than that found in any other blood sample?
You would do well to not judge the rest of the world with the lens of American performances. Being driven everywhere, fast food and pop, video games, mySpace, poker, skateboarding and other more lucrative sports aren't productive for developing phenomenal distance runners. Really, we're doing pretty well in spite of the conditions.