That is based on the incorrect assumption that there are a fixed number of jobs. That isn't true. It's more dynamic than that. The total number of jobs expands or contracts based on all kinds of factors. There are plenty of scenarios where we have more population in the US, but lower unemployment and vice versa.
For an industry to operate it requires employees at a range of skill levels. For example, a software company still requires people to clean the place and pack the boxes and whatnot. Virtually all people that grow up in the US are overqualified for some of the lowest end jobs. Just having even completed junior high school qualifies someone to work, for example, at a McDonalds, but there are many jobs below that skill level that our economy needs filled. For example, picking fruit is a lower skill job. If a person who is qualified for a job at McDonalds is working as a fruit picker, that is inefficient. It's a waste of resources, it produces less tax revenue, lowers our nation's quality of living, etc.
But, in order to sustain the good jobs, we need to fill the bad jobs. Without people picking fruit, the fruit company won't exist so it won't be able to hire truck drivers and accountants and sales people and whatnot. We have two options for how to fill those kinds of jobs. We can either force citizens to do them somehow or we can let people from elsewhere who are less skilled do them and try to create more skilled jobs for the more skilled folks. With other people taking care of the low skill jobs we have the labor pool to allow industries to expand and we free up our more skilled workers for better jobs. That approach makes a lot more sense for us. We should be focused on moving forwards, not holding ourselves back by clinging to low end jobs.