It's such a huge space though. Americans are kind of weird. If you would've put 20% of the population in Arabs or something inside Poland or Romania, you would have riots and armed revolution. Americans just got used to running away, isolating in gated communities, going to the suburbs, to Florida, there's always another promise-land to run to.
Also when you start dealing with sub-Saharan migrants or radicalized local blacks, Mexicans suddenly don't seem so bad.
Trump was definitely elected on his promise to do something radical and illiberal and stop migration, but he didn't really do any of that. Just look at Hungary or Poland. Poland mobilized the army and built fences in weeks when Lukashenko tried to bully it with thousands of imported Middle Easterners. They left them to freeze in the woods and pushed them back as if they were an invading army, regardless of moaning from the West EU. Hungary has fences at the border and removes illegal migrants on the spot. Croats and Serbs often have vigilante groups that patrol the border and beat/send back migrants. Greece has fences now too and openly rejects the Turkish attempts to use migration as a weapon. Italy is trying to criminalize the NGOs that "save" migrants in the Med and more (although it's harder and harder to do anything as you move West).
You can find a LOT of excuses about why the US is not doing any of this in a significant manner, but the truth is, IMO, that in average, the US doesn't care about anything "native", cheap imported labor is far more important. The whites with financial means will isolate. The poor whites will suffer, and that's the price to pay for the "melting pot".