One reason is population growth. Our current system is based on the assumption of an ever growing labor force to fund things like social security, medicare, fund our massive debt, and evrything else we want the government to spend on. In their current form, these systems will break down in the face of population decline. Since existing Americans are having fewer kids and trending downward, immigration is the only way to sustain the model.

This doesn't neccisarily.mean the is the best, or even desireable, way to structure society, but I also think the political system is dysfunctional to the point major change is currently impossible

I didnt down vote you by the way. Just throwing out a counter point to consider