> If I can hire someone remotely in San Antonio, why can't I hire someone remotely in São Paulo as a result?
Because the person in Brazil would be able to steal your IP and you'd have no financial, legal, or emotional recourse since they're across an international border. Big tech is continuing to find this out the hard way when countries like China and India somehow clone their products in record time.
By hiring in the same country you reside in, you have access to your legal system to protect you and your business interests. And it can also protect your own ego as well (if you have one).
And for the USA specifically, it is also really about the people here. Tech is a surprisingly human industry, and Americans have the humanity and grit to pull it off.
> pay a lower salary
Nothing is stopping tech from doing this right now to American workers. Remember that those higher paying firms do false positive hiring - they reject way more people than they pull in. If you do the reverse and adjust your operations to account for more workers that "aren't good but aren't bad either", you'd probably perform even better than offshoring.