I largely agree with this in the context of many nations not only America. There are basically no elite level American talent going into the marathon until they are older and have done a few years of post collegiate professional track cycles. So, by the time they hit the roads for the marathon they may have great seed development from their track years but they are older and have less “shelf-life” left in them to get enough experience over the marathon to maximize their potential.
The piece I agree about from above is that it is a numbers game, to a large extent, and the less talent you have competing in an event the less consistent you will be. As has been mentioned by others, America has had notable successful marathon racers but they are consistent because there is a huge cohort of racers at that level, instead of a common upper tier of ability they are the peaks. Either way, you get the numbers up and the results are almost inevitable but right now, and I can’t see why this would change any time soon, all the potential is being poured into track distances.