It is quite remarkable. My best guess is that it just has to do with a difference in training methods and attitudes. American elite high schoolers live in constant fear of 'burnout' or doing anything that might dampen their 'long-term potential'; the attitude seems to be to depend on extreme talent and do the sort of minimum necessary training volume to flesh that out. This seems to lead to most elite high schoolers circling around 40 to 70mpw. There are the occasional 70+, but this doesn't seem to include the highest level of talent.
Meanwhile I'd imagine (and, yes, this is more or less baseless speculation) that the top tier of their young talent is routinely pursuing volumes in excess of 100mpw, and therefore reaching a much higher standard.
If you roll back the clock to the 60's or 70's, the age of Eric Hulst and Gerry Lindgren and all that, you see hordes of American high schoolers putting in 100-140mpw and attaining times much closer to the global peak. (For example, 8:40 2mile might be rather pedestrian these days, but at the time was rather impressive for anyone, let alone the highschoolers achieving it). We are only just returning to a point where we can consistently produce high school runners reaching those 8:40ish times, but obviously the world has since moved on without us.
But yea I'd say its really just a training, and predominantly a volume, thing. We have the talent, but you don't hear about Solinsky or Ritz -esque 90+mpw in high school these days. Instead you have outrageous talents like Knight or Fisher or what have you, running maybe 50mpw, playing soccer, or whatever. The training volume and level of specialization just aren't there. It makes sense that in Japan they are there much more consistently and so we see them making the most of their youth talent.