In Australia, immigration is used to increase the population, which is used to increase gdp. The problem is that infrastructure is not improved to meet the growing population, and the increase in gdp is not enough to go around for the new people, so everybody in the middle and lower class gets a smaller part of the gdp. Cities where you could afford a nice house in a quiet area become concrete jungles where people are stacked on top of each other in apartments. House prices (for the same type of house people were used to living in) increase massively due to the increase in demand.

I am assuming something similar happens in America. The increase in population just means a decrease in quality of life for the existing people.