If the US has a comparative advantage in producing these goods, they will produce them themselves. As businesses in a country will change their production/reallocate resources between different industries over time, those industries that are profitable will survive.

It's sad that poor countries export the food out of the country. Poverty is also very hard to understand and explain. One theory that can explain a bit, is that most poor countries must import real capital like machines, oil etc. To afford this, they have save less money, which again gives them less economic growth and it's spiraling downwards. Research has found that many countries have managed to improve their economic growth by making export of real capital illegal (one of the main points of the Solow Growth Model).