Think about the extremes:
Extreme #1) Your country exports nothing and imports as much as it possibly can. Over decades of time, your country builds up piles and piles of goods which all have value. Plus your country produces its own goods of value and keeps them all. This value makes your country quite wealthy. If this continued forever, your country would eventually have everything in the world, and the other countries would have nothing. Your country can just keep printing money and recieve all the goods you want.
Extreme #2) Your country imports nothing and exports as much as it possibly can. As your country produces its goods, it exports them all. Over time, your country would have nothing of value since it exported everything away. Your country would have no jewelry, no cars, no computers, no houses, no food, no clothes since they were all exported out. You have nothing at all except a pile of printed money that has no value since you won't purchase anything).
Ok which extreme sounds better? Having everything or having nothing?
Obviously neither extreme can happen in reality - but this really shows why America has more wealth, more goods, more of almost everything. Since it is a net importer. Countries that are net exporters have less of almost everything.