What do you mean "America is full?" Our population density is 182 of ~250 on a global scale. We have more space, money, and food per person than almost every other country on the planet.
And just try driving in any major prosperous city at rush hour or riding the subway. Or renting an apartment in SF or NY. We can't expand housing or infrastructure in those cities and we're admitting millions of foreigners overfilling them even further.