There is a reason Mantz & Young beat a bunch of 2:06/2:05 and faster guys at the Olympics. They are actually very good marathoners, the incentives just don't align for Americans to run those fast times.
If you are trying to support your family on running, you can't turn down the larger appearance fees at American majors of which 2 of 3 are on slow courses. Even if you go to Chicago on a perfect weather day, you still have to think about your contract bonus and extra prize money for finishing top American, making you more risk averse. Running while afraid of blowing up, is not the way to push the absolute limits of your capabilities and potentially surprise yourself with a huge PB.
For Biya Simbassa, the incentives were actually aligned to do something special in Europe. His relatively slow PB and lower profile due to a lack of NCAA and professional track success, meant he probably couldn't command large appearance fees at Chicago and New York, so he went to Europe to take a chance and improve his PB. Now that he has run the 4th fastest American time ever, I bet his earnings at those US-based marathons will go WAY UP. It will be interesting to see if he goes back to Europe now.