This isn't 1700's America where a bunch of drunk Irish people are setting up shop to make America Great Again. We are an established country.
American has an overabundance of people with little or no skills, it doesn't need to import more of them. Just look at how many liberal university grads got *** toilet paper degree's and now are demanding government assistance. With so many educated unskilled welfare recipients in the system why on earth would the USA need to import uneducated unskilled welfare recipients?
Now when it comes to skilled immigrants we can talk a bit more. Americans shouldn't be directly in competition with someone from India or Pakistan for a job while that person is still in India or Pakistan. The cost of living and tax disparity alone would destroy the US professional working class, in fact it has been for decades. There is already a surplus of American professional workers, generally in the entry level ranks. The "hard to find" workers you see companies complaining about are in the experienced ranks and it's become "hard to find" because companies no longer wish to train and grow their workforce due to becoming reliant on outsourcing and H1B replacements. If a place routinely fill it's lower echelon with foreign workers, it's going to be genuinely impossible to find an experienced American senior worker.