One idea that crosses my mind is that two guys don't make a "ton." Maybe there are more and you're just referencing two. I don't keep close enough tabs on college running to know but the best way to prove this point would be to compare improvements, or lack thereof, of comparable populations of former US and Kenyan collegiate runners.
Another idea is that you're talking about college guys with decent but not great track times who have moved to the marathon. We have had US guys who've done that. Bill Rodgers and Frank Shorter immediately come to mind as do Tom Fleming, Amby Burfoot, Dick Beardsley Greg Meyer, and Benji Durden. But all of these guys were really focused on the marathon. They raced at other distances but that was really all secondary to racing marathons. And the older guys in that group generally raced multiple marathons a year for several years.
No one in the US is really doing that today.
We've pretty much gone back to the days where the best guys stay on the track until they've "peaked" there and then move to the marathon. And even when someone does focus on the marathon they race the distance very sparingly.
Consider what you've written about Chemlany. His first marathon was 2:13 and it took him seven tries to get to sub 2:08. He reminds me of Ron Hill whose first marathon was 2:24 in 1961. It took him three years to get under 2:15 and five more after that to get under 2:10. Similarly, Durden did not burst into the marathon world as a 2:10 guy. It took him years of racing the distance and tinkering with his training to get under 2:10.
How many US guys with decent track times do that now? How many of them even run seven marathons in their enitire marathoning career? Most of our guys work their way to marathoning very cautiously and then give it up if they don't get a good time in their first two or three tries.