Through trade (and the threat of trade sanctions) we have enormous influence on foreign governments. The US in aggregate has more leverage over China than (say) Idaho does over California. Why should the citizens of Idaho accept competition from Californians?
If it's because CA and ID both roll up into the federal government, then don't we all roll up into the UN?
>we have vast amounts of our own poor we underserve
We do. The welfare state is too weak, we don't do nearly enough redistribution, it's unconscionable that we lack national health insurance, it's unconscionable that we let neighborhood wealth determine school quality and make college cost so much, etc. But raising the price of goods by locking out foreign competition is regressive: the made-in-America premium hurts most when you have the least to spend.