You’ve asked a very good question.
And the Duggars are considered an oddity...and yet their family would have been completely ordinary in 1850.
America had no significant immigration from 1770 to 1840.
Yet it grew exponentially. Senator Seward even used this growth rate as a justification for the admission of California in his speech to the Senate.
His calculations were almost dead on:
Well-established calculations in political arithmetic enable us to say that the aggregate population of the nation now is – - – 22,000,000
That 10 years hence it will be -30,000,000
That 20 years hence it will be -38,000,000
That 30 years hence it will be – 50,000,000
That 40 years hence it will be – 64,000,000
That 50 years hence it will be – 80,000,000
That 100 years hence, that is, in the year 1950, it will be – 200,000,000
equal nearly to one-fourth of the present aggregate population of the globe, and double the population of Europe at the time of the discovery of America
The Great Wave immigrants were brought in because they replaced the millions never born because 700,000 men died in the "Civil" war, so that was 7 million babies who never existed if the 10 children per family number is used.
Obviously some of those men did have children but the point is this: the immigrants of the 1880's - 1920's simply filled a vacuum created by the war dead.
And we have a similar vacuum now: the 55 million killed by abortion.
It's not an accident that this number is close to the number of immigrants over the last 40 years. They are simply taking the place of those never born - an entire generation.
What rational nation kills itself off and then hands over its land and wealth to foreigners?