I think everyone is right. Yes, Rupp is a once in a generation talent. Period. But, the US is not East Africa. We have other sports. We have careers. I know so many sub-4:25 HS milers and even a few sub 4:10 college milers that gave up running in college for pre-med, engineering etc.
A Kenyan running 4:20 as a junior with a scholarship to come to America will run as if their life depended on it.
An American with a 4:20 in H.S. with an academic scholarship to Stanford might not run at all.
If you are a 13:59 5K guy there is no real incentive to train. You might genetically able to get to 13:30 and maybe run a 2:10 marathon but are you as a low tier D1 athlete going to give up jobs at Goldman Sachs and Nike to train for 5 years for the 10% chance you’ll make it to that point? No, if you’re lucky, you’ll keep running on the side and be proud of the 15:59 5K at your local shamrock run and take home the Wal Mart gift card.
When Europeans train in Kenya they become better. Look at Wanders and those twins from NZ. But are you willing to give up everything?!?! How many NCAA champions are willing to do that? I don’t blame them.
Also, there is tons of talent in other sports. I had guys in H.S. run 4:59 off no training to sub 16:00 5K on 30 miles a week as a junior in a few months. I am not saying this guy could have been Rupp, but he at least could have been a D2/low D1 runner.
As for Japan, they place prestige and emphasis on road running. It is special there.