Well, I think there's a simple explanation -- it's because we have a gun culture.
In a lot of these countries, the only way you can legally get your hands on a gun is if it's in competition training.
The guns that they use in the Olympics are largely of the calibers that we trust our kids with. That may seem to cloud my argument. If we're shooting .22lrs when we're just out of diapers, why wouldn't we be the best as adults? Well, it's because that's never what we tell our kids to stick with. As kids can handle guns that deliver more punch, we tend to upgrade.
Americans train with guns all the time; but, we train for different things.
Yeah, we Americans enjoy plinking and such. But, training for us looks like firearm retention. It looks like running courses in home invasion scenarios.
Namely, our gun culture has a more practical component.
The guns that they use in the Olympics are great for controlled environments. They don't recoil. The technique is different.
American gun culture and training is about self-defense, hunting, and keeping the government in check. That takes different tools and different training.
If Olympic shooting competition looked more like picking up a 2011 and clearing a house, America would smoke everybody.