As it stands, we have vast amounts of our own poor we underserve, but it's less sexy to say you support our own poor, functionally illiterate than say you support some people in a country where people made different political and economic decisions.
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.
Japan would speak to your position otherwise. Some people say it's foolhardy for them to have expensive fruit and vegetables, when they could import cheaper. On the other hand those farmers are self sustaining within their system.
I'm with Seattle anti WTO spirit of '99.
you could always lower sales tax and raise income tax
That's where democratic influence stops, beyond which we can't control. Before globalization became a thing, workers commonly had a union, bargaining rights, and a political wing that fought for them. Now that we've entered a global economy, we can't get that leverage anymore.
You're trying to re-install economic partitions at the national level which currently aren't there; "that's how we do things" isn't an argument. "How we organize ourselves politically" with respect to economics has been as an integrated global civilization since the mid to late 20th century.
If economic partitioning is an unqualified good, why not go much smaller than country scale? If it's not an unqualified good, what makes national borders optimal? Doesn't it seem a little bit suspicious that a country grown over hundreds of years of political wrangling just happens to the mathematically optimal economy size?