Certainly there is a lot of swirling complexity around this topic…but the bare bones reasons are thus:
The United States is the world’s largest economy and just flat out consumes a ton of stuff. And it wants that stuff to be cheap. And it wants that stuff to be available year round even for seasonal items. So it imports…bigly.
And even if the United States truly wanted to make and grow and produce everything it consumes in the United States…it wouldn’t be able to without expanding it’s population via an increased birthrate and/or increased immigration. There just isn’t a sufficient contemporary workforce to do the job.
There’s your quick hit on the economics of world trade 001.