"Food" is a mass noun. Without an article, it could mean all the food in the world, or it could mean any quantity of food, eg:
I haven't had any food today.
Humans need food.
There is a food shortage.
You need a definite article when you are referring to specific food. In your example, it is only the food in America that you want to refer to. It is essentially the same as saying "American food".
You could say "Food is good in America", but that doesn't specifically mean American food.
Consider this example:
Cold drinks taste good in hot weather.
This is a generalisation about all cold drinks, but the statement is really about the conditions you drink it in, not the drink itself, because you could drink the same drink in cold weather and make a different statement about it. The same is true if you say "food is good in America". It is like saying any food is good if you eat it in America. If you buy some food in Mexico and take it accross the border to eat it in America, it doesn't change the properties of the food. So, to be clear that you specifically mean the food that is in America, you need the article.